Exhaust gas purification device

ABSTRACT

There is provided an exhaust gas purification device ( 22 ) comprising a substrate used for purifying components contained in an exhaust gas discharged from an engine. The substrate has partitions ( 54 ) which define passages ( 50,51 ) and are formed of porous material having fine pores each having a predetermined average size. The end portions of the adjacent partitions ( 54 ) defining each of part of the passages ( 50,51 ) of the substrate are partially connected to each other such that the end portions ( 52,53 ) are tapered toward the outside of the substrate. The tapered end portions partially close the end opening of the corresponding passage and form a small hole ( 55,56 ) defined by the tips thereof. The size of each small hole ( 55,56 ) is smaller than the cross sectional area of the corresponding passage ( 50 ) and larger than the sizes of the fine pores of the partitions ( 54 ).

TECHNICAL FIELD

The invention relates to an exhaust gas purification device.

BACKGROUND ART

A particulate filter for collecting particulates contained in exhaustgas discharged from an engine is known. The filter has a honeycombstructure formed of porous material. Further, the filter has a pluralityof passages, some of them being closed at their upstream end by plugs,and remaining of them being closed at their downstream end by plugs. Inthis filter, the exhaust gas passes through partitions defining thepassages, and thereafter flows out of the filter.

In this type of the filter, as the exhaust gas flows out of the filterafter passing through the partitions, the filter has a high particulatecollection ratio. However, the passages of the filter are closed by theplugs, and therefore, the productivity of the filter is low and the costfor producing the filter is high.

Further, as shown in FIG. 3A, as the exhaust gas hits against the plugs,the exhaust gas does not flow smoothly into the passages of the filter.In addition, when the exhaust gas flows near the upstream plugs, theexhaust gas flows with turbulence, and thus the exhaust gas does notflow smoothly into the passages of the filter. Further, as shown in FIG.3B, when the exhaust gas flows near the downstream plugs, the exhaustgas flows with turbulence, and thus the exhaust gas does not flowsmoothly out of the filter. For these reasons, the filter has a largepressure loss.

The purpose of the invention is to provide an exhaust gas purificationdevice having a particulate filter having a small pressure loss.

DISCLOSURE OF INVENTION

In the first invention, there is provided an exhaust gas purificationdevice comprising a substrate used for purifying components contained inan exhaust gas discharged from an engine, the substrate havingpartitions which define passages and are formed of porous materialhaving fine pores each having a predetermined average size, the endportions of the adjacent partitions defining each of part of thepassages of the substrate being partially connected to each other suchthat the end portions are tapered toward the outside of the substrate,the tapered end portions partially closing the end opening of thecorresponding passage and forming a small hole defined by the tipsthereof, and the size of each small hole being smaller than the crosssectional area of the corresponding passage and larger than the sizes ofthe fine pores of the partitions.

In the second invention, according to the first invention, the endportions of the adjacent partitions defining each of part of thepassages of the substrate are partially connected to each other at theirupstream ends such that the end portions are tapered toward the outsideof the substrate, and the end portions of the adjacent partitionsdefining each of remaining passages of the substrate are partiallyconnected to each other at their downstream ends such that the endportions are tapered toward the outside of the substrate.

In the third invention, according to the second invention, the taperedend portions and the remaining partitions carry oxidation material foroxidizing the particulates, and the amount of the oxidation materialcarried by each upstream tapered end portion per unit volume is largerthan that carried by each downstream tapered end portion per unitvolume.

In the fourth invention, according to the first invention, the endportions of the adjacent partitions defining each of part of thepassages of the substrate are partially connected to each other at theirupstream ends such that the end portions are tapered toward the outsideof the substrate, and the end portions of the adjacent partitionsdefining each of remaining passages of the substrate are connected toeach other at their downstream ends such that the end portions aretapered toward the outside of the substrate and the downstream endopening of the passage is completely closed.

In the fifth invention, according to the first invention, the endportions of the adjacent partitions defining each of part of thepassages of the substrate are partially connected to each other at theirdownstream ends such that the end portions are tapered toward theoutside of the substrate, and the end portions of the adjacentpartitions defining each of remaining passages of the substrate areconnected to each other at their upstream ends such that the endportions are tapered toward the outside of the substrate and theupstream end opening of the passage is completely closed.

In the sixth invention, according to the first invention, the substrateis used as a particulate filter arranged in an exhaust gas passage of anengine for collecting particulates contained in an exhaust gasdischarged from an engine.

In the seventh invention, according to the sixth invention, the taperedend portions carry oxidation material for oxidizing the particulates.

In the eighth invention, according to the seventh invention, the amountof the oxidation material carried by each tapered end portion at itsupstream surface per unit are is larger than that at its downstreamsurface per unit area.

In the ninth invention, according to the seventh invention, a processfor increasing the temperature of the filter is performed.

In the tenth invention, according to the seventh invention, the filtercarries a NOx carrying agent to take in and carry the NOx therein whenexcessive oxygen exists therearound, and to discharge the carried NOxtherefrom when the concentration of the oxygen decreases.

In the eleventh invention, according to the seventh invention, thefilter carries a precious metal catalyst.

In the twelfth invention, according to the eleventh invention, theoxidation material is an active oxygen production agent to take in andcarry the oxygen when excessive oxygen exists therearound, and todischarge the carried oxygen therefrom in the form of active oxygen whenthe concentration of the oxygen decreases, and the active oxygenproduction agent discharges the active oxygen therefrom when theparticulates adhere to the filter to oxidize the particulate adhering tothe filter by the active oxygen.

In the thirteenth invention, according to the twelfth invention, theactive oxygen production agent comprises one of an alkali metal, analkali earth metal, a rare earth and a transition metal.

In the fourteenth invention, according to the twelfth invention, theactive oxygen production agent comprises one of an alkali metal and analkali earth metal having an ionization tendency higher than that ofcalcium.

In the fifteenth invention, according to the twelfth invention, the airfuel ratio of at least part of the exhaust gas flowing into the filteris temporarily made rich to oxidize the particulates adhering to thefilter.

In the sixteenth invention, according to the sixth invention, anoxidation means for oxidizing components contained in the exhaust gas isarranged in the exhaust gas passage of the engine upstream of thefilter.

In the seventeenth invention, according to the sixteenth invention, theoxidation means is an oxidation catalyst.

In the eighteenth invention, according to the sixteenth invention, theoxidation means is a NOx catalyst to carry the NOx when the lean exhaustgas flows thereinto and to reduce the carried NOx when the rich exhaustgas flows thereinto.

In the nineteenth invention, according to the sixth invention, the sizeof each small hole of the filter at the low temperature region of thefilter is larger than that at the high temperature region of the filter.

In the twentieth invention, according to the nineteenth invention, thelow temperature region is the peripheral region of the filter, and thehigh temperature region is the central region of the filter.

In the twenty-first invention, according to the nineteenth invention,the cross sectional area of each passage of the filter at the lowtemperature region of the filter is larger than that at the hightemperature region of the filter.

In the twenty-second invention, according to the sixth invention, thecross sectional area of each passage of the filter at the lowtemperature region of the filter is larger than that at the hightemperature region of the filter.

In the twenty-third invention, according to the twenty-second invention,the low temperature region is the peripheral region of the filter, andthe high temperature region is the central region of the filter.

In the twenty-fourth invention, according to the twenty-secondinvention, the size of each small hole of the filter at the lowtemperature region of the filter is larger than that at the hightemperature region of the filter.

In the twenty-fifth invention, according to the sixth invention, anexhaust gas purification means for purifying components contained in theexhaust gas is arranged in the exhaust gas passage of the enginedownstream of the filter.

In the twenty-sixth invention, according to the twenty-fifth invention,the exhaust gas purification means is a NOx catalyst to carry the NOxwhen the lean exhaust gas flows thereinto, and to reduce the carried NOxwhen at least the generally stoichiometric exhaust gas flows thereinto.

In the twenty-seventh invention, according to the twenty-fifthinvention, the exhaust gas purification means is an additionalparticulate filter which can oxidize the particulates contained in theexhaust gas.

In the twenty-eighth invention, according to the twenty-fifth invention,the filter is arranged at least near the exhaust manifold.

In the twenty-ninth invention, according to the twenty-fifth invention,the device further comprises a bypass passage which extends from theengine exhaust gas passage between the filter and the exhaust gaspurification means to the exhaust gas passage of the engine downstreamof the exhaust gas purification means to bypass the exhaust gaspurification means, and a switch valve for switching the flow of theexhaust gas into the exhaust gas purification means and into the bypasspassage, the filter carries a SOx carrying agent to carry the SOx whenthe lean exhaust gas flows thereinto, and to release the carried SOxwhen at least the generally stoichiometric exhaust gas flows thereintoand the temperature of the SOx carrying agent has a temperature higherthan a SOx release temperature, the switch valve is positioned such thatthe exhaust gas flows into the exhaust gas purification means when theSOx is not released from the SOx carrying agent, and is positioned suchthat the exhaust gas flows into the bypass passage when the SOx isreleased from the SOx carrying agent.

In the thirtieth-invention, according to the twenty ninth invention, acatalyst for oxidizing the components contained in the exhaust gas isarranged in the bypass passage.

In the thirty-first invention, according to the first invention, thesubstrate is arranged in an exhaust gas passage of an engine, thesubstrate carrying a hydrocarbon collection agent for collectingunburned hydrocarbon contained in an exhaust gas discharged from anengine, and a hydrocarbon purification catalyst for purifying unburnedhydrocarbon, the hydrocarbon collection agent collects unburnedhydrocarbon when the agent has a temperature lower than a hydrocarbonrelease temperature, and releases the collected unburned hydrocarbontherefrom when the agent has a temperature higher than the hydrocarbonrelease temperature, the hydrocarbon purification catalyst purifiesunburned hydrocarbon when the catalyst has a temperature higher than ahydrocarbon purification temperature, the hydrocarbon releasetemperature is set such that the unburned hydrocarbon is released fromthe hydrocarbon collection agent when the hydrocarbon purificationcatalyst has a temperature lower than the hydrocarbon purificationtemperature.

In the thirty-second invention, there is provided a method for producinga substrate used for purifying components contained in an exhaust gasdischarged from an engine, the substrate having a plurality of exhaustgas passages defined by partitions formed of porous material, the endportions of the partitions defining each of part of the exhaust gaspassages being partially connected to each other at one end of theexhaust gas passage such that the end portions are tapered toward theoutside of the substrate and define a small hole by the tips thereof,the end portions of the partitions defining each of the remainingexhaust gas passages being partially connected to each other at theother end of the exhaust gas passage such that the end portions aretapered toward the outside of the substrate and define a small hole bythe tips thereof, wherein the method comprises a step of gathering andconnecting the end portions of the partitions defining each exhaust gaspassage to be closed at its end opening, and a step of forming a smallhole defined by the tips of the end portions defining each exhaust gaspassage to be closed at its end opening, each small hole having a sizesmaller than the area of the end opening of the corresponding exhaustgas passage and larger than the average sizes of the fine pores of thepartitions.

In the thirty-third invention, according to the thirty-second invention,the gathering and connecting step and the small hole forming step aresimultaneously performed.

In the thirty-fourth invention, according to the thirty-third invention,the gathering and connecting step and the small hole forming step aresimultaneously performed by pressing a device having a plurality ofprojections and pins arranged between the projections onto the end faceof the substrate.

In the thirty-fifth invention, according to the thirty-second invention,first, the gathering and connecting step is performed, and then thesmall hole forming step is performed.

In the thirty-sixth invention, according to the thirty-fifth invention,in the small hole forming step, the tips of the end portions connectedto each other are shaved to form the small hole.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show a particulate filter of the invention;

FIGS. 2A and 2B show a part of the filter of the invention;

FIGS. 3A and 3B show a part of the filter of the prior art;

FIGS. 4A and 4B show a honeycomb structure;

FIGS. 5A and 5B show a die;

FIGS. 6A and 6B show an action of the oxidization of the particulates;

FIGS. 7A, 7B and 7C show an action of deposition of the particulates;

FIGS. 8 shows a relationship between the amount of the particulatespurified by the oxidation and the temperature of the filter;

FIGS. 9A and 9B show the filter of the second embodiment;

FIGS. 10A and 10B show the filter of the third embodiment;

FIG. 11 shows an engine provided with the filter of the invention;

FIG. 12 shows a flowchart for controlling the engine operation;

FIG. 13 shows the engine provided with the exhaust gas purificationdevice of the fourth embodiment;

FIGS. 14A and 14B show an oxidization catalyst of the fourth embodiment;

FIGS. 15A and 15B show the filter of the fifth embodiment;

FIGS. 16A and 16B show the filter of the sixth embodiment;

FIG. 17 shows the engine provided with the exhaust gas purificationdevice of the seventh embodiment;

FIGS. 18A and 18B show a main particulate filter of the seventhembodiment;

FIGS. 19A-19C show an action of the oxidation of the particulates by themain filter;

FIG. 20 shows the engine provided with the exhaust gas purificationdevice of the modified seventh embodiment;

FIG. 21 shows the engine provided with the exhaust gas purificationdevice of the eighth embodiment;

FIG. 22A shows ratios of the discharged NOx and SOx;

FIG. 22B shows total amounts of the discharged NOx and SOx;

FIG. 23 shows the exhaust gas purification device of the ninthembodiment;

FIGS. 24A and 24B show the filter produced by the second filterproduction method;

FIGS. 25A and 25B show a closure device used in the second filterproduction method;

FIGS. 26A and 26B show the second filter production method;

FIGS. 27A and 27B show the closure device used in the third filterproduction method;

FIGS. 28A and 28B show the closure device used in the third filterproduction method;

FIGS. 29A and 29B show the third filter production method;

FIG. 30 shows the closure device used in the fourth filter productionmethod;

FIG. 31 shows the closure device used in the fifth filter productionmethod;

FIGS. 32A and 32B show the details of the closure device shown in FIG.31;

FIGS. 33A and 33B show the closure device used in the sixth filterproduction device; and

FIGS. 34A and 34B show the seventh filter production method.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be explained by referring to the drawings. FIGS. 1Aand 1B show an end view and a longitudinal cross sectional view of thefilter, respectively. As shown in FIGS. 1A and 1B, the filter 22 has ahoneycomb structure, and a plurality of exhaust gas passages 50, 51extending parallel to each other.

The exhaust gas passages of the filter 22 comprise exhaust gas inflowpassages 50. At the downstream end region, each inflow passage 50 has across sectional area which is made smaller than that of its remainingregion by a corresponding tapered wall portion 52. Further, the exhaustgas passages of the filter 22 comprise exhaust gas outflow passages 51.At the upstream end region, each outflow passage 51 has a crosssectional area which is made larger than that of its remaining region bya corresponding tapered wall portion 53.

Each downstream tapered wall portion 52 is formed by gathering andconnecting the downstream end portions of partitions 54 defining thecorresponding exhaust gas inflow passage 50 to each other. On the otherhand, each upstream tapered wall portion 53 is formed by gathering andconnecting the upstream end portions of partitions 54 defining thecorresponding exhaust gas outflow passage 51 to each other.

Each inflow passage 50 has a small hole 55 at the tip of thecorresponding downstream tapered wall portion 52. Each small hole 55 hasa cross sectional area smaller than that of the corresponding inflowpassage 50. On the other hand, each outflow passage 51 has a small hole56 at the tip of the corresponding upstream tapered wall portion 53.Each small hole 56 has a cross sectional area smaller than that of thecorresponding outflow passage 51. In other words, the downstream endopenings of some of the exhaust gas passages 50 are partially closed bythe downstream tapered wall portions 52 to define the small holes 55therein. On the other hand, the upstream end openings of remainingexhaust gas passages 51 are partially closed by the upstream taperedwall portions 53 to define the small holes 56 therein.

According to the present invention, the exhaust gas passages 50, 51 ofthe filter 22 are alternatively positioned and the thin partitions 54are positioned between the passages 50,51. In other words, the inflowpassage 50 is enclosed by four outflow passages 51, and the outflowpassage 51 is enclosed by four inflow passages 50. Therefore, the crosssectional area of one of two adjacent exhaust gas passages of the filter22 is decreased at its downstream end region by the correspondingdownstream tapered wall portion 52, and the cross sectional area of theother exhaust gas passage is decreased at its upstream end region by thecorresponding upstream tapered wall portion 53.

The filter 22 is formed of porous material such as cordierite.Cordierite has fine pores each having a predetermined average size.Therefore, as shown by an arrow in FIG. 1B, the exhaust gas flowing intothe inflow passages 50 flows into the adjacent outflow passages 51through the fine pores of the surrounding partitions 54. As the taperedwall portions 52, 53 are also formed of the same material as thepartitions 54, the exhaust gas flows into the outflow passages 51through the fine pores of the upstream tapered wall portions 53 as shownin FIG. 2A and flows out of the inflow passages 50 through the finepores of the downstream tapered wall portions 52 as shown in FIG. 2B.

Further, the exhaust gas flows into the outflow passages 51 through theupstream small holes 56, and flows out of the inflow passages 50 throughthe downstream small holes 55.

Each hole 55,56 has a size larger than the average sizes of the finepores of the tapered wall portions 52,53. Further, the downstream smallholes 55 have generally the same sizes as each other, and the upstreamsmall holes 56 have generally the same sizes as each other. Furthermore,the downstream small holes 55 may have generally the same as or,different sizes from those of the upstream small holes 56.

If the small holes 55,56 have large sizes, the filter 22 has a smallpressure loss and a low particulate collection ratio. Contrary to this,if the small holes 55,56 have small sizes, the filter 22 has a largepressure loss and a high particulate collection ratio. According to thepresent invention, the size of the hole 55,56 is determined such thatthe pressure loss and the particulate collection ratio of the filter 22are suitably balanced. Further, according to the present invention, thesize of the hole 55,56 is determined such that the amount of theparticulates flowing out of the filter 22 is kept smaller than anallowed amount. The amount of the particulates flowing out of the filter22 can be calculated on the basis of the amount of the particulatesflowing into the filter 22 per unit time and the particulate collectionratio of the filter 22.

In the present invention, the particulate collection ratio and thepressure loss of the filter 22 can be easily changed by changing thesize of the holes 55,56 in accordance with the target particulatecollection ratio of the filter 22.

Each upstream tapered wall portion 53 conically converges toward itsupstream end such that the cross sectional area of the correspondingoutflow passage 51 decreases continuously. Therefore, the upstream endof each inflow passage 50 defined by the corresponding upstream taperedwall portion 53 conically diverges toward its upstream end such that thecross sectional area of the corresponding inflow passage 50 increasescontinuously. According to this structure, the exhaust gas smoothlyflows into the filter 22 contrary to the case that the entrances of theinflow passages are constituted as shown in FIG. 3A.

In the filter as shown in FIG. 3A, each inflow passage is closed at itsupstream end by a plug 72. In this case, as shown by the referencenumber 73, the exhaust gas hits against the plugs 72, and the filter hasa large pressure loss. In addition, as shown by the reference number 74,when the exhaust gas flows near the plugs 72, the exhaust gas flows withturbulence around the entrances of the inflow passages, and thus theexhaust gas does not smoothly flow into the inflow passages. Thus, thefilter has a large pressure loss.

On the other hand, in the filter 22 of the invention, as shown in FIG.2A, the exhaust gas flows into the inflow passages 50 withoutturbulence. Therefore, according to the present invention, the exhaustgas smoothly flows into the filter 22, and thus the filter 22 has asmall pressure loss.

In the filter shown in FIGS. 3A and 3B, as the exhaust gas hits againstthe plugs 72 and flows with turbulence around the plugs 72, theparticulates easily deposit on the upstream end surfaces of the plugs 72and the wall surfaces of the partitions adjacent thereto. However, inthe filter 22 of the present invention, as each upstream tapered wallportions 53 has a conical shape, the tapered wall portion 53 has noupstream end surface which the exhaust gas hit against, and the exhaustgas does not flow with turbulence around the upstream end surface of thetapered wall portion 53. Therefore, according to the present invention,many particulates hardly deposit in the filter 22 at its upstreamregion, and the pressure loss of the filter 22 hardly increases.

On the other hand, each downstream tapered wall portion 52 conicallyconverges toward its downstream end such that the cross sectional areaof the corresponding inflow passage 50 decreases continuously.Therefore, the downstream end of each outflow passage 51 defined by thecorresponding downstream tapered wall portion 52 conically divergestoward its downstream end such that the cross sectional area of thecorresponding outflow passage 51 increases continuously. According tothis structure, the exhaust gas smoothly flows out of the filter 22contrary to the case that the exits of the outflow passages areconstituted as shown in FIG. 3A.

In the filter shown in FIG. 3B, each outflow passage is closed at itsdownstream end by a plug 70, and extends straight to its exit.Therefore, turbulence 71 occurs around the exits of the outflowpassages. In this case, the exhaust gas does not smoothly flow out ofthe outflow passages.

On the other hand, in the filter 22 of the invention, as shown in FIG.2B, the exhaust gas flows out of the exit of the outflow passages 51without turbulence. Therefore, according to the present invention, theexhaust gas smoothly flows out of the filter 22, and thus the filter 22has a small pressure loss.

As long as each tapered wall portion continuously converges toward theoutside of the filter 22, the tapered wall portion may be in the formother than the conical form, such as a quadrangular pyramid or a sixsided pyramid.

It is preferred that the filter has potentially a small pressure loss.Further, an engine operation control is designed in consideration of thepotential pressure loss of the filter. Therefore, if the pressure lossof the filter increases from the potential pressure loss during theengine operation, the performance of the engine decreases. Regarding thefilter, it is important that the filter has a small potential pressureloss and the pressure loss of the filter is kept around the potentialpressure loss even if the pressure loss of the filter increases in useof the filter.

According to the present invention, as the partitions 54 defining theupstream end region of the exhaust gas passages 50,51 of the filter 22are tapered, the exhaust gas hardly flows with turbulence when theexhaust gas flows into the exhaust gas passages 50,51, and thus thepressure loss of the filter 22 is potentially small.

Further, according to the present invention, since the partitions 54defining the upstream end regions of the exhaust gas passages 50,51 ofthe filter 22 are tapered, the particulates hardly deposit on thesurfaces of the tapered wall portions 52,53. In other words, in use ofthe filter 22, the particulates hardly deposit on the surfaces of thetapered wall portions 52,53, and thus the exhaust gas flowing into theexhaust gas passages hardly flows with turbulence by the depositedparticulates. Therefore, even if the pressure loss of the filterincreases in use thereof, the pressure loss of the filter hardlyincreases to a value considerably larger than the potential pressureloss.

Other than the particulates, the exhaust gas contains incombustibleinorganic residuals (ash) produced by the burning of the fuel.Therefore, the ash flows into the filter 22, and deposits therein.

When the amount of the ash depositing in the inflow passages 50increases, the pressure loss of the filter 22 increases. As explainedabove, in use of the filter 22, it is important that the pressure lossof the filter 22 is kept around the potential pressure loss even if thepressure loss of the filter 22 increases. To this end, the amount of thedepositing ash must be small. Further, it is preferred that the ashdepositing in the inflow passages 50 is removed.

According to the present invention, the small holes 55 are formed in thedownstream tapered wall portions 52, and thus the ash flowing into theinflow passages 50 can flow out through the downstream small holes 55.Therefore, the ash hardly deposits in the inflow passages 50, and thusthe pressure loss of the filter 22 hardly increases to a valueconsiderably larger than the potential pressure loss even if thepressure loss of the filter 22 increases.

Further, when the amount of the ash depositing in the inflow passages 50increases, the quantity of the exhaust gas passing through the smallholes 55,56 increases. Therefore, the amount of the ash newly depositingin the inflow passages 50 decreases, and thus the pressure loss of thefilter 22 hardly increases to a value considerably larger than thepotential pressure loss even if the pressure loss of the filter 22increases.

Further, when the amounts of the ash and the particulates depositing ineach inflow passage 50 increase, and then the pressure in the inflowpassage 50 increases, the increased pressure moves the ash depositing inthe inflow passage 50 toward its downstream region, and finallydischarges the ash through the corresponding downstream small hole 55.Therefore, the pressure loss of the filter 22 hardly increases to avalue considerably larger than the potential pressure loss even if thepressure loss of the filter 22 increases. In addition, as the ashdepositing in the inflow passages 50 is discharged from the filter 22 bythe pressure in the inflow passages 50, the number of the operations todischarge the ash from the filter 22 is reduced.

Further, when the amounts of the ash and the particulates depositing inthe inflow passages 50 increase, the exhaust gas does not easily passthrough the partitions 54, and thus the pressures in the inflow passages50 increase. At this time, the quantity of the exhaust gas passingthrough the small holes 55,56 increases. Therefore, the pressure loss ofthe filter 22 hardly increases to a value considerably larger than thepotential pressure loss even if the pressure loss of the filter 22increases.

Further, when many particulates deposit in the filter 22, and burn atonce, the filter 22 may be melted by the heat derived from the burningof the particulates. However, according to the present invention, manyparticulates hardly deposit in the filter 22. Therefore, the filter ishardly melted by the heat derived from the burning of the particulates.

The invention may be applied to an exhaust gas purification filterarranged in the exhaust passage of the engine for collecting specificcomponents contained in the exhaust gas, or to an exhaust gaspurification catalyst arranged in the exhaust passage of the engine forpurifying specific components contained in the exhaust gas.

A first method for producing a filter of the present invention will beexplained. First, a cylindrical honeycomb structure 80 shown in FIG. 4is extruded from porous material such as cordierite. Next, a die 90shown in FIG. 5 is pressed onto one of the end faces of the structure80.

As shown in FIG. 5A, the die 90 has a plurality of conical projections91. FIG. 5B shows one of the projections 91. The die 90 is pressed ontoone end face of the structure 80 such that each projection 91 isinserted into a corresponding exhaust gas passage 50. As a result, fourend portions of four adjacent partitions defining each exhaust gaspassage 51 are gathered toward each other, and then are partiallyconnected to each other to form a tapered wall portion 53 and a smallhole 56.

Regarding the other end face of the structure 80, the similar processesare performed.

Next, the structure 80 is dried. Next, the structure 80 is baked. As aresult, the filter 22 of the present invention is obtained. In this way,according to the present invention, the ends of the passages 50,51 arenarrowed by a very simple method comprising the step of pressing the die90 onto the end faces of the structure 80.

The step of pressing the die 90 onto the end faces of the structure 80may be performed after the structure 80 is dried. Otherwise, after thebaked structure 80 is softened at its end regions, the die 90 may bepressed onto the softened end portions of the structure 80. In thiscase, thereafter, the end portions of the structure 80 are baked again.

In the present invention, in use of the filter 22, the particulates donot easily deposit on the upstream tapered wall portions 53. However, insome cases, the particulates may deposit on the wall portions 53. Inthis case, in use of the filter 22, the pressure loss of the filter 22increases. As explained above, in use of the filter 22, it is importantto prevent the pressure loss of the filter from increasing to a valueconsiderably larger than the potential pressure loss. To this end, it isnecessary to remove the particulates from the filter 22.

According to the present invention, an oxidation material for removingthe particulates by oxidation is applied on the upstream tapered wallportions 53. According to this, the particulates collected by thetapered wall portions 53 are continuously removed by oxidation and manyparticulates hardly deposit on the upstream tapered wall portions 53.Therefore, in use of the filter 22, the pressure loss of the filter 22hardly increases to a value considerably larger than the potentialpressure loss even if the pressure loss of the filter 22 increases inuse of the filter 22.

In this way, according to the present invention, a problem is solved,which specially derives from the structure of the porous tapered wallportions of the upstream ends of the outflow passages 51, that is, inwhich the pressure loss of the filter increases to a value considerablylarger than the potential pressure loss during use of the filter.

In the present invention, the oxidation material is applied to theentire of the filter 22, that is, the partitions 54 and the downstreamtapered wall portions 52 other than the upstream tapered wall portions53. Further, in the present invention, the oxidation material is appliedto the interior wall surfaces defining the fine pores of the upstreamand downstream tapered wall portions 53,52 and the partitions 54 otherthan the exterior wall surfaces thereof. Furthermore, in the presentinvention, the amount of the oxidation material applied to the upstreamtapered wall portions 53 per unit volume is larger than those applied tothe partition 54 or the downstream tapered wall portions 52.

The exhaust gas more easily passes through the tapered wall portions52,53 than the partitions 54. That is, the quantity of the exhaust gaspassing through the tapered wall portions 52,53 per unit surface area islarger than that passing through the partitions 54 per unit surfacearea. Therefore, commonly, the amount of the particulates depositing onthe tapered wall portions 52,53 is larger than that depositing on thepartitions 54, and thus the tapered wall portions 52,53 are more easilyclosed by the particulates than the partitions 54.

Opposed to this, according to the present invention, the amount of theoxidation material applied to the tapered wall portion 52,53 per unitvolume is larger than that applied to the partition 54 per unit volume.According to this, the amount of the particulates removed by oxidationon each tapered wall portion 52,53 per unit time is larger than that oneach partition 54 per unit time. Therefore, many particulates hardlydeposit on the tapered wall portions 52,53.

The exhaust gas is difficult to pass through the tapered wall portions52,53 if much oxidation material is applied to the tapered wall portions52,53. Therefore, the exhaust gas generally uniformly passes through thetapered wall portions 52,53 and the partitions 54. Thus, manyparticulates hardly deposit on the tapered wall portions 52,53. Further,the tapered wall portions 52, 53 and the partitions 54 are efficientlyused for collecting the particulates.

The amount of the particulates depositing on each tapered wall portion52,53 at its upstream wall surface is larger than that at its downstreamwall surface. That is, the tapered wall portions 52,53 are more easilyclosed at their upstream wall surfaces by the particulates than at theirdownstream wall surfaces. According to the present invention, the amountof the oxidation material applied to the upstream wall surface of eachtapered wall portion 52,53 per unit volume is larger than that appliedto the downstream wall surface thereof. According to this, the finepores of the tapered wall portions 52,53 are hardly closed by theparticulates.

The oxidation material applied to the filter will be explained indetail. In the present invention, a carrier layer is formed of thematerial such as alumina on the surrounding wall surfaces of the exhaustgas passages 50,51, i.e., the entire of the both sides of the partitions54 and the tapered wall portions 52,53. Precious metal catalyst andactive oxygen production agent are carried on the carrier layer. Theagent takes and carries the oxygen when the excess of the oxygen existsaround the agent, and discharges the carried oxygen therefrom in theform of an active oxygen when the concentration of the oxygen around theagent decreases. In the first embodiment, the oxidation material isconstituted by the active oxygen production agent.

In the first embodiment, a platinum (Pt) is used as the precious metalcatalyst, and at least one of the material selected from an alkali metalsuch as potassium (K), sodium (Na), lithium (Li), cesium (Cs) orrubidium (Rb), an alkali earth metal such as barium (Ba), calcium (Ca)or strontium (Sr), or a rare earth such as lanthanum (La), yttrium (Y)or Cerium (Ce), a transition metal such as iron (Fe), or a carbon familyelement such as Tin (Sn), is used as the active oxygen production agent.

It is preferred that an alkali metal or an alkali earth metal having anionization tendency larger than calcium, that is, potassium, lithium,cesium, rubidium, barium or strontium is used as the active oxygenproduction agent.

The action of removal of the particulates by the filter will beexplained in the case that platinum and potassium are carried on thecarrier layer. Note that the action of removal of the particulates bythe filter carrying other precious metal and other alkali metal, oralkali earth metal, or rare earth, or transition metal is generally thesame as that explained below.

For example, in the case that the engine is a type of the compressionignition engine in which the fuel burns under an excess of the oxygen inthe combustion chamber, the exhaust gas flowing into the filter 22contains excessive oxygen. That is, in the case that the air fuel ratioof the mixture in the combustion chamber 5 is referred to as the airfuel ratio of the exhaust gas, in the compression ignition engine, theair fuel ratio of the exhaust gas is lean. Further, nitrogen monoxide(NO) is produced in the combustion chamber 5 of the compression ignitionengine, and thus the exhaust gas contains NO. Furthermore, the fuelcontains a sulfur constituent (S). The sulfur constituent reacts withthe oxygen in the combustion chamber 5 and becomes sulfur dioxide (SO₂).Therefore, the exhaust gas contains SO₂. Thus, the exhaust gascontaining the excessive oxygen, NO, and SO₂ flows into the inflowpassages 50 of the filter 22.

As explained above, the exhaust gas contains the excessive oxygen and,thus, if the exhaust gas flows into the inflow passages 50 of the filter22, as shown in FIG. 6A, the oxygen (O₂) adheres to the surface of theplatinum in the form of O₂ ⁻ or O²⁻. On the other hand, the NO in theexhaust gas reacts with the O₂ ⁻ or O²⁻ on the surface of the platinumto become NO₂ (2NO+O₂→2NO₂). Next, part of the produced NO₂ is oxidizedon the platinum and is adsorbed to the active oxygen production agent61, and thus is carried in the agent 61 in the form of nitrate ions NO₃⁻. Otherwise, part of the produced NO₂ is oxidized on the platinum andis absorbed and diffuses in the agent 61, and thus is carried in theagent 61 in the form of nitrate ions (NO₃ ⁻). As shown in FIG. 6A, thenitrate ions NO₃ ⁻ bond with potassium to produce potassium nitrate(KNO₃).

On the other hand, as explained above, the exhaust gas also containsSO₂. This SO₂ is carried in the active oxygen production agent 61 by amechanism similar to that of NO. That is, the oxygen (O₂) adheres to thesurface of the platinum in the form of O₂ ⁻ or O²⁻. The SO₂ in theexhaust gas reacts with the O₂ ⁻ or O²⁻ on the surface of the platinumto become SO₃. Next, part of the produced SO₃ is oxidized on theplatinum and is adsorbed to the agent 61, and thus is carried in theagent 61 in the form of sulfate ions (SO₄ ²⁻). Otherwise, part of theproduced SO₃ is oxidized on the platinum and is absorbed and diffuses inthe agent 61, and thus is held in the agent 61 in the form of sulfateions (SO₄ ²⁻). The sulfate ions (SO₄ ²⁻) bond with the potassium toproduce potassium sulfate (K₂SO₄).

On the other hand, particulates comprised of mainly carbon (C), that is,soot, are produced in the combustion chamber 5. Therefore, the exhaustgas contains particulates. The particulates contact and adhere to thesurface of the carrier layer, for example, the surface of the activeoxygen production agent 61 as shown in FIG. 6B when the exhaust gasflows in the inflow passages 50 of the filter 22 or passes through thepartitions 54.

If the particulates 62 adhere to the surface of the active oxygenproduction agent 61 in this way, the concentration of oxygen at thecontact surface between the particulate 62 and the agent 61 falls. Ifthe concentration of oxygen falls, a difference in concentration occurswith the inside of the high oxygen concentration active oxygenproduction agent 61, and therefore the oxygen in the agent 61 movestoward the contact surface between the particulate 62 and the agent 61.As a result, the potassium nitrate (KNO₃) formed in the agent 61 isbroken down into potassium, oxygen, and NO. The oxygen moves toward thecontact surface between the particulate 62 and the agent 61, while theNO is released from the surface or the inside of the agent 61 to theoutside. The NO released to the outside is oxidized on the downstreamside platinum and is again carried by adsorption or absorption in theagent 61.

On the other hand, at this time, the potassium sulfate (K₂SO₄) formed inthe active oxygen production agent 61 is also broken down intopotassium, oxygen, and SO₂. The oxygen moves toward the contact surfacebetween the particulate 62 and the agent 61, while the SO₂ is releasedfrom the surface or the inside of the agent 61 to the outside. The SO₂released to the outside is oxidized on the downstream side platinum andagain carried by adsorption or absorption in the agent 61. Note that,since the potassium sulfate is stable and does not easily dissolve, thepotassium sulfate does not easily release the active oxygen comparedwith the potassium nitrate.

As explained above, the active oxygen production agent 61 produces andreleases the active oxygen by the reaction with the oxygen when theagent 61 absorbs the NOx therein in the form of the nitrate ions (NO₃⁻). Similarly, as explained above, the agent 61 produces and releasesthe active oxygen by the reaction with the oxygen when the agent 61absorbs the SO₂ therein in the form of the sulfate ions (SO₄ ²⁻).

The oxygen moving toward the contact surface between the particulate 62and the active oxygen production agent 61 is the oxygen broken down fromcompounds such as potassium nitrate (KNO₃) or potassium sulfate (K₂SO₄).The oxygen broken down from these compounds has an unpaired electron andthus is the active oxygen having an extremely high reactivity.Therefore, the oxygen moving toward the contact surface between theparticulate 62 and the agent 61 becomes the active oxygen. Similarly,the oxygen produced by the reaction of the NOx and the oxygen in theagent 61 or the reaction of the SO₂ and the oxygen in the agent 61becomes the active oxygen. If the active oxygen contacts the particulate62, the particulate 62 is oxidized without emitting a luminous flame ina short period (from several seconds to several minutes) and theparticulate 62 is completely removed. Therefore, the particulates hardlydeposit on the filter 22.

In the prior art, when the particulates depositing in layers on thefilter burn, the filter becomes red hot and burns along with a flame.This burning along with a flame does not continue unless the temperatureis high. Therefore, to continue the burning along with a flame, thetemperature of the filter must be maintained high.

As opposed to this, in the present invention, the particulate 62 isoxidized without emitting a luminous flame as explained above. At thistime, the surface of the filter 22 does not become red hot. That is, inthe present invention, the particulate 62 is removed by oxidation at alow temperature compared to the prior art. Therefore, the action ofremoval of the particulate 62 by oxidation without emitting a luminousflame according to the present invention is completely different fromthe action of removal of particulate by burning along with a flame.

The higher the temperature of the filter 22, the more active theplatinum and the active oxygen production agent 61 become. Therefore,the higher the temperature of the filter 22, the amount of theparticulates removable by oxidation without emitting a luminous flame onthe filter 22 per unit time increases.

The solid line in FIG. 8 shows the amount G of the particulatesremovable by oxidation without emitting a luminous flame per unit time.The abscissa of FIG. 8 shows the temperature TF of the filter 22. If theamount of particulates flowing into the filter per unit time is calledthe inflowing particulate amount M, in the state that the inflowingparticulate amount M is smaller than the amount G of particulatesremovable by oxidation, that is, in the region I of FIG. 8, when theparticulates contact the filter 22, all of the particulates flowing intothe filter 22 are removed by oxidation successively in a short time(from several seconds to several minutes) without emitting a luminousflame on the filter 22.

As opposed to this, in the state that the inflowing particulate amount Mis larger than the amount G of particulates removable by oxidation, thatis, in the region II of FIG. 21, the amount of the active oxygen is notsufficient for successive oxidation of all of the particulates. FIGS. 7Ato 7C show the state of oxidation of particulates in this case.

That is, in the state that the amount of active oxygen is not sufficientfor successive oxidation of all of the particulates, if the particulate62 adheres to the active oxygen production agent 61 as shown in FIG. 7A,only part of the particulate 62 is oxidized. The portion of theparticulate not sufficiently oxidized remains on the carrier layer ofthe active oxygen particulate agent 61. Next, if the state of aninsufficient amount of active oxygen continues, the portions of theparticulates not oxidized successively remain on the carrier layer. As aresult, as shown in FIG. 7B, the surface of the carrier layer is coveredby the residual particulate portion 63.

When the surface of the carrier layer is covered by the residualparticulate portion 63, the platinum does not easily oxidize the NO andSO₂, and the active oxygen production agent 61 does not easily releasethe active oxygen, and thus the residual particulate portion 63 is notoxidized and easily remains as it is. As a result, as shown in FIG. 7C,other particulates 64 successively deposit on the residual particulateportion 63. That is, the particulates deposit in layers.

If the particulates deposit in layers in this way, the particulates willnot be oxidized by the active oxygen. Therefore, other particulatessuccessively deposit on the particulate 64. That is, if the inflowingparticulate amount M continues to be larger than the amount G ofparticulates removable by oxidation, the particulates deposit in layerson the filter 22 and therefore unless the temperature of the exhaust gasis made higher or the temperature of the filter 22 is made higher, it isno longer possible to cause the deposited particulates to ignite andburn.

As explained above, in the region I of FIG. 8, the particulates areoxidized in a short time without emitting a luminous flame on the filter22. In the region II of FIG. 8, the particulates deposit in layers inthe filter 22. Therefore, to prevent the particulates from depositing inlayers in the filter 22, the inflowing particulate amount M must bemaintained smaller than the amount G of the particulates removable byoxidation at all times.

As can be understood from FIG. 8, in the filter 22 of the presentinvention, the particulates can be oxidized even if the temperature TFof the filter 22 is considerably low. Therefore, it is possible tomaintain the inflowing particulate amount M and the filter temperatureTF such that the inflowing particulate amount M is normally maintainedsmaller than the amount G of the particulates removable by oxidation. Ifthe inflowing particulate amount M is maintained smaller than the amountG of the particulates removable by oxidation at all time, theparticulates hardly deposit in the filter 22 and the pressure loss ofthe filter 22 hardly increases.

On the other hand, as explained above, in the state that theparticulates deposit in layers on the filter 22, the active oxygen doesnot easily oxidize the particulates even when the inflowing particulateamount M becomes smaller than the amount G of the particulate removableby oxidation. However, when the portions of the particulates notoxidized begin to remain, that is, the amount of the depositingparticulates is smaller than an allowed limit, if the inflowingparticulate amount M becomes smaller than the amount G of theparticulates removable by oxidation, the remaining portions of theparticulates are oxidized and removed by the active oxygen withoutemitting a luminous flame.

The filter of the second embodiment will be explained. FIGS. 9A and 9Bshow the filter of the second embodiment. FIGS. 9A and 9B show an endview and a longitudinal cross sectional view of the filter,respectively. The structure of the filter of the second embodiment isbasically the same as that of the first embodiment.

In the second embodiment, similar to the first embodiment, a small hole55 is formed in the tip of each downstream tapered wall portion 52.However, no small hole is formed in the tips of the upstream taperedwall portions 53. That is, the inflow passages 53 are completely closedby the upstream tapered wall portions 53. Therefore, the particulatecollection ratio of the filter of the second embodiment is larger thanthat of the first embodiment.

In the second embodiment, even if the ash and the particulates depositin the inflow passages 50, and then the exhaust gas does not easily passthrough the partitions 54, the exhaust gas newly flowing into the inflowpassages 50 can flow out of the filter 22 through the downstream smallholes 55. Therefore, according to the second embodiment, the pressureloss of the filter 22 hardly increases to a value considerably largerthan the potential pressure loss thereof.

Further, in the second embodiment, in the state that the amount of theash and the particulates depositing in the inflow passages 50 increases,and then the pressure in the inflow passages 50 increases, the ash isdischarged from the filter 22 through the downstream small holes 55 bythe pressure in the inflow passages 50. Therefore, according to thesecond embodiment, the amount of the ash depositing on the filter 22 ismaintained small at all times and, thus, it is not necessary to performany special process for removing the ash from the filter 22 many times.

Furthermore, in the second embodiment, in the state that manyparticulates deposits in the inflow passages 50, and thus the exhaustgas does not easily pass through the partitions 54, the particulatesnewly flowing into the inflow passages 50 flow out of the filter 22through the downstream small holes 55. Therefore, the amount of theparticulates depositing in the filter 22 is maintained smaller than aconstant amount. Thus, many particulates hardly burn in the filter 22 atonce and, thus, the filter 22 is hardly melted by the heat derived fromthe burning of the particulates.

Further, the filter 22 of the second embodiment carries the oxidationmaterial therein, and thus the depositing particulates are successivelyoxidized away. Therefore, in the state that many particulates deposit inthe filter 22, and then some of the particulates are not collected bythe filter 22 and flow out of the filter 22, that is, the particulatecollection ratio of the filter 22 is small, the depositing particulatesare successively oxidized away by the oxidation material, and thus someof the particulates newly flowing into the filter 22 are collected bythe filter 22. Therefore, the amount of the particulates not collectedby the filter 22 and flowing out thereof hardly considerably increases.

Immediately before the ash depositing in the inflow passages 50 isdischarged from the filter 22 by the pressure in the inflow passages 50,the pressure in the inflow passages 50 may temporarily considerablyincreases. In the first embodiment, the exhaust gas newly flowing intothe filter 22 flows into the outflow passages 51 through the upstreamsmall holes 56, and then flows out of the filter 22. Therefore, thepressure in the inflow passages 50 hardly further increases.

However, in the second embodiment, until the ash is discharged from thefilter 22, the pressure in the inflow passages 50, that is, the pressureloss of the filter 22 may continue to increase. Therefore, it ispreferred to employ the filter of the second embodiment in the case thatit is allowed that the pressure loss of the filter temporarily increasesto the relatively high level, or in the case that the high particulatecollection ratio is required of the filter.

In the second embodiment, the outflow passages 51 may be completelyclosed by the upstream tapered wall portion 53 by pressing the die 90onto the upstream end face of the honeycomb structure 80 to an extentlarger than that of the first method for producing the filter.

The filter of the third embodiment will be explained. FIGS. 10A and 10Bshow the filter of the third embodiment. FIGS. 10A and 10B show an endview and a longitudinal cross sectional view of the filter. Thestructure of the filter of the third embodiment is basically the same asthat of the first embodiment.

In the first embodiment, similar to the first embodiment, a small hole56 is formed in the tip of each upstream tapered wall portion 53.However, no small hole is formed in the tip of each downstream taperedwall portion 52. That is, the inflow passages 50 are completely closedby the downstream tapered wall portions 52. Thus, the particulatecollection ratio of the filter of the third embodiment is larger thanthat of the first embodiment.

In the third embodiment, in the state that the ash and the particulatesdeposit in the inflow passages 50, and thus the exhaust gas does noteasily pass through the partitions 54, the exhaust gas newly flowinginto the filer 22 may flow out of the filter 22 through the upstreamsmall holes 56. Therefore, according to the third embodiment, even ifthe pressure loss of the filter increases, the pressure loss of thefilter hardly increases to a value considerably larger than thepotential pressure loss thereof.

Further, in the third embodiment, as explained above, in the case thatthe ash and the particulates deposit in the inflow passages 50, and thusthe exhaust gas does not pass through the partitions 54, much exhaustgas flows out of the filter 22 through the upstream small holes 56 andthe outflow passages 51. That is, part of the ash newly flowing into thefilter is discharged from the filter 22 through the upstream small holes56 and the outflow passages 51. Therefore, it takes long time until theamount of the ash depositing in the inflow passages 50 becomes largerthan the allowed amount. Thus, it is not necessary to perform a specialprocess for removing the ash depositing in the filter 22 many times.

Further, in the third embodiment, as explained above, in the case thatthe ash and the particulates deposit in the inflow passages 50 and thusthe exhaust gas does not easily pass through the partitions 54, muchexhaust gas flows into the outflow passages 51 through the upstreamsmall holes 56 and flows out of the outflow passages 51. That is, partof the particulates newly reaching the filter 22 flows into the outflowpassages 51 through the upstream small holes 56 and flows out of theoutflow passages 51. Therefore, it takes long time until the amount ofthe particulates depositing in the inflow passages 50 becomes largerthan an allowed amount. Thus, the filter 22 is hardly melted by the heatderived from the burning of the particulates.

The filter 22 of the third embodiment carries the oxidation materialtherein. Therefore, by the time that the amount of the particulatesdepositing in the inflow passages 50 becomes larger than the allowedamount, the depositing particulates are oxidized and removed by theoxidation material. Thus, the amount of the depositing particulateshardly becomes larger than the allowed amount.

It is preferred to employ the filter of the third embodiment in the casethat the exhaust gas contains a small amount of the ash and the largepressure loss of the filter is allowed, or in the case that the highparticulate collection ratio is required of the filter. Further, in thethird embodiment, the inflow passages 50 are completely closed bypressing the die 90 onto the downstream end face of the honeycombstructure 80 to the extent larger than that in the first method forproducing the filter.

A control of the engine provided with the filter of the presentinvention will be explained. FIG. 11 shows a compression ignition typeengine provided with the filter of the present invention. Note that thefilter of the present invention may be applied to a plug ignition typeengine.

Referring to FIG. 11, 1 shows an engine body, 2 a cylinder block, 3 acylinder head, 4 a piston, 5 a combustion chamber, 6 an electricallycontrolled fuel injector, 7 an intake valve, 8 an intake port, 9 anexhaust valve, and 10 an exhaust port. The intake port 8 is connectedthrough a corresponding intake branch pipe 11 to a surge tank 12. Thesurge tank 12 is connected through an intake duct 13 to a compressor 15of an exhaust turbocharger 14.

A throttle valve 17 driven by a stepping motor 16 is arranged in theintake duct 13. An intercooler 18 for cooling the intake air passing theintake duct 13 is arranged around the intake duct 13. In the engineshown in FIG. 11, an engine cooling water is supplied to the intercooler18, and cools the intake air.

The exhaust port 10 is connected to an exhaust turbine 21 of the exhaustturbocharger 14 through an exhaust manifold 19 and an exhaust pipe 20.The outlet of the exhaust turbine 21 is connected to a casing 23 housinga particulate filter 22 through an exhaust pipe 20 a.

The exhaust manifold 19 is connected to the surge tank 12 through an EGRpassage 24. An electrically controlled EGR control valve 25 is arrangedin the EGR passage 24. Further, an EGR cooler 26 for cooling the EGR gaspassing through the EGR passage 24 is arranged around the EGR passage24. In the engine shown in FIG. 11, the engine cooling water is suppliedto the EGR cooler 26 and cools the EGR gas.

On the other hand, each fuel injector 6 is connected to the fuelreservoir, that is, a common rail 27 through a fuel supply tube 6 a.Fuel is supplied to the common rail 27 from an electrically controlledvariable discharge fuel pump 28. Fuel supplied to the common rail 27 issupplied to the fuel injectors 6 through the fuel supply tubes 6 a. Afuel pressure sensor 29 for detecting the fuel pressure in the commonrail 27 is attached to the common rail 27. The amount of discharge offuel from the fuel pump 28 is controlled such that the fuel pressure inthe common rail 27 is maintained at a target fuel pressure on the basisof the output signal of the fuel pressure sensor 29.

An electronic control unit 30 is comprised of a digital computer and isprovided with a read only memory (ROM) 32, a random access memory (RAM)33, a microprocessor (CPU) 34, an input port 35, and an output port 36.The output signal of the fuel pressure sensor 29 is input to the inputport 35 through the corresponding AD converters 37. A temperature sensor39 for detecting the exhaust gas temperature is attached to the filter22. The output signal of the temperature sensor 39 is input to the inputport 35 through the corresponding AD converter 37.

A load sensor 41 is connected to an accelerator pedal 40. The sensor 41generates an output voltage proportional to the amount of depression Lof the accelerator pedal 40. The output voltage of the sensor 41 isinput to the input port 35 through a corresponding AD converter 37.Further, a crank angle sensor 42 for generating an output pulse eachtime a crankshaft rotates by, for example, 30° is connected to the inputport 35. On the other hand, the output port 36 is connected to the fuelinjectors 6, the stepping motor 16, the EGR control valve 25 and thefuel pump 28 through corresponding drive circuits 38.

As explained above, in the state that the particulates deposit in layersin the filter 22, even if the amount M of the inflowing particulatesbecomes smaller than the amount G of the particulates removable byoxidation, the active oxygen does not easily oxidize the particulates.In particular, immediately after the engine starts up, the filtertemperature TF is low. At this time, the amount M of the inflowingparticulates is larger than the amount G of the particulate removable byoxidation. In the state that the portions of the particulates notoxidized start to remain, in other words, in the state that the amountof the depositing particulates is smaller than an allowed upper limit,if the amount M of the inflowing particulates becomes smaller than theamount G of the particulates removable by oxidation, the remainingportions of the particulates are oxidized and removed by the activeoxygen without emitting a luminous flame.

Therefore, in the present invention, the amount M of the inflowingparticulates and the filter temperature TF are maintained such that theamount M of the inflowing particulates is smaller than the amount G ofthe particulates removable by oxidation. In addition, in the presentinvention, the amount M of the inflowing particulates and the filtertemperature TF are maintained such that the remaining portions 63 of theparticulates hardly cover the surface of the carrier layer as shown inFIG. 7B even if the amount M of the inflowing particulates temporarilybecomes larger than the amount G of the particulates removable byoxidation, in other words, such that the amount of the particulatesdepositing in layers is maintained smaller than an allowed limit, andthe particulates may be oxidized and removed when the amount M of theinflowing particulates becomes smaller than the amount G of theparticulates removable by oxidation.

However, even if the amount M of the inflowing particulates and thefilter temperature TF are controlled as explained above, theparticulates may deposit in layers in the filter 22. In this case, theparticulates depositing in the filter 22 may be oxidized and removedwithout emitting a luminous flame by temporarily making the air fuelratio of part or entire of the exhaust gas rich.

That is, when the air fuel ratio of the exhaust gas is maintained leanfor a while, much oxygen adheres to the platinum. Therefore, thecatalytic action of the platinum decreases. However, if the air fuelratio of the exhaust gas is made rich to decrease the concentration ofoxygen in the exhaust gas, the oxygen is removed from the platinum, andthus the catalytic action of the platinum increases. Therefore, when theair fuel ratio of the exhaust gas is made rich, much active oxygen iseasily discharged from the active oxygen production agent 61 to theoutside at once. Thus, the depositing particulates are reformed by theactive oxygen at once to the easily oxidizable state, and then theparticulates burn away. Therefore, when the air fuel ratio of theexhaust gas is made rich, the amount G of the particulates removable byoxidation increases.

In this case, the air fuel ratio of the exhaust gas may be made richwhen the particulates deposit in layers in the filter 22. Otherwise, theair fuel ratio of the exhaust gas may be periodically made richindependent of whether the particulates deposit in layers.

For example, the air fuel ratio of the exhaust gas is made rich bycontrolling the quantity of the fuel injected from the injector 6 suchthat the average air fuel ratio of the mixture in the combustion chamber5, while the valve lifts of the throttle valve 17 and the EGR controlvalve 25 are controlled such that the EGR rate (the quantity of the EGRgas/(the quantity of the intake air+the quantity of the EGR gas)) ismaintained larger than 65 percent when the engine load is small.

As explained above, in the case that the particulates are oxidized awayby making the air fuel ratio of the exhaust gas rich when theparticulates deposit in the filter 22 and are not easily oxidized away,the filter 22 of the present invention has an advantage that hydrocarbon(HC) hardly adheres to the upstream area of the filter 22.

That is, if the air fuel ratio of the exhaust gas is made rich, thehydrocarbon flows into the filter 22. At this time, the hydrocarboneasily adheres to the upstream area of the filter 22. As the temperatureat the upstream area of the filter 22 is lower than that at itsdownstream area, the hydrocarbon adheres to the filter 22 at itsupstream area and is not easily consumed. Thus, the hydrocarbon depositson the upstream area of the filter 22. However, in the presentinvention, since the filter 22 carries much oxidation material at itsupstream area, the hydrocarbon is consumed and is hardly depositsthereon. Therefore, the hydrocarbon hardly closes the upstream area ofthe filter 22.

FIG. 12 shows an example of the routine of the engine operation controlexplained above. Referring to FIG. 12, first, at step 100, it is judgedif the average air fuel ratio of the mixture in the combustion chamber 5should be made rich. When it is not necessary to make the average airfuel ratio of the mixture in the combustion chamber 5 rich, the valvelift of the throttle valve 17 is controlled at step 101, the valve liftof the EGR control valve 25 is controlled at step 102, and the quantityof the fuel injected from the injector 6 is controlled at step 103 suchthat the amount M of the inflowing particulates becomes smaller than theamount G of the particulates removable by oxidation.

On the other hand, when it is judged that the average air fuel ratio ofthe mixture in the combustion chamber 5 should be made rich at step 100,the valve lift of the throttle valve 17 is controlled at step 104 andthe valve lift of the EGR control valve 25 at step 105 such that the EGRrate becomes larger than 65 percent, and the quantity of the fuelinjected from the injector 6 is controlled at step 106 such that theaverage air fuel ratio of the mixture in the combustion chamber 5becomes rich.

Fuel or lubrication oil contains calcium (Ca). Therefore, the exhaustgas contains calcium. Calcium produces calcium sulfate (CaSO₄) in thepresence of SO3. The calcium sulfate is a solid and will not break downby heat even at a high temperature. Therefore, if calcium sulfate isproduced, the calcium sulfate closes the fine pores of the filter 22. Inthis case, the exhaust gas does not easily pass through the filter 22.

In this case, if an alkali metal or an alkali earth metal having anionization tendency higher than that of calcium, for example potassium(K), is used as the active oxygen production agent 61, the SO3 diffusingin the agent 61 bonds with the potassium to become potassium sulfate(K₂SO₄). On the other hand, the calcium does not bond with the SO3, andthen passes through the partitions 54 of the filter 22 and flows intothe outflow passages 50. Therefore, there is no longer any clogging ofthe fine pores of the partitions 54. Thus, as explained above, it ispreferable to use an alkali metal or an alkali earth metal having anionization tendency higher than calcium, that is, potassium, lithium,cesium, rubidium, barium, and strontium, as the active oxygen productionagent 61.

The present invention may be applied to a filter comprising onlyprecious metal such as platinum carried on the carrier layer formedtherein. In this case, the solid line denoting the amount G of theparticulates removable by oxidation slightly moves to the right side inFIG. 8. Further, in this case, NO₂ or SO₃ carried on the surface of theplatinum produces the active oxygen. Furthermore, as the active oxygenproduction agent, a catalyst may be used, which adsorbs and carries NO₂or SO₃, and produces the active oxygen from the carried NO₂ or SO₃.

The fourth embodiment will be explained. As shown in FIG. 13, in thefourth embodiment, an oxidation catalyst 22 a is arranged in the exhaustpassage downstream of the outlet portion of the exhaust turbine 21 andupstream of the filter 22. The catalyst 22 a is housed in a casing 23 a.A temperature sensor 39 a for detecting the temperature of the catalyst22 a is attached to the catalyst 22 a. The output signal of the sensor39 a is input into the input port 37 through the corresponding ADconverter 39 a.

The oxidation catalyst 22 a is, for example, formed by coating thinlayers of alumina on the carrier formed of ceramics such as cordierite,or heat resistant steel, and then applying the precious metal catalystto the alumina layer. The precious metal catalyst has an oxidationability, and thus the catalyst 22 a may strongly oxidize specificconstituents, in particular, hydrocarbon and carbon monoxide (2CO+O→CO₂,HC+O₂→CO₂+H₂O) away.

The oxidation catalyst 22 a carries has an amount of the precious metalcatalyst per unit surface area larger than that carried by the filter22. In the catalyst 22 a of the fourth embodiment, one or more ofplatinum, palladium and rhodium is used as the precious metal catalyst.Further, in the fourth embodiment, in addition to the precious metalcatalyst, an oxygen storage agent such as cerium or nickel for absorbingand releasing the oxygen may be carried on the alumina carrier.Furthermore, in the fourth embodiment, in addition to the precious metalcatalyst, a stabilization agent such as barium, lanthanum, or zirconiumfor preventing the change of the alumina and the precious metal catalystby heat.

As shown in FIG. 14, the oxidation catalyst 22 a comprises a honeycombcarrier. Therefore, the catalyst 22 a has a plurality of exhaust gaspassages 22 b therein, which are defined by partitions 22 c and extendparallel to each other. Further, the inlet and outlet openings of theexhaust gas passages 22 b of the catalyst 22 a are completely open. Thatis, the catalyst 22 a is a monolith type catalyst. In the catalyst 22 a,the exhaust gas is not forced to pass through the partitions 22 c, andthus the catalyst 22 a has a small pressure loss. Therefore, thecatalyst 22 a arranged upstream of the filter 22 increases the pressureloss of the exhaust gas purification device to only a small extent.

The exhaust gas contains particulates such as soot and Soluble OrganicFraction (SOF) therein. The fuel is smothered in the state that theoxygen is not sufficient in the combustion chamber. The soot is producedfrom the smothered fuel and mainly consists of carbon (C). On the otherhand, the fuel and the hydrocarbon contained in the engine oil areevaporated by the high temperature in the combustion chamber and thendeposit to become particulate matters by the decreasing of thetemperature in the combustion chamber. The SOF is the depositingparticulate matter and mainly consists of hydrocarbon.

If the atmosphere surrounding the precious metal catalyst carried in thefilter 22 is oxidative, the catalyst strongly oxidizes the materialcontained in the exhaust gas. Therefore, if the engine is a compressionignition type engine, the lean exhaust gas is discharged therefrom, andthus the precious metal catalyst demonstrates a strong oxidizablity.Therefore, the SOF contained in the exhaust gas is oxidized away by theoxidizability of the catalyst (C_(m)H_(n)+O₂→CO₂+H₂O). Of course, theSOF contained in the exhaust gas is oxidized away by the active oxygenreleased from the active oxygen production agent 61(C_(m)H_(n)+O*→CO₂+H₂O).

However, depending on the engine operation state, the concentration ofthe SOF in the exhaust gas discharged from the combustion chamber 5 maytemporarily increase. In this case, the concentration of the SOF in theexhaust gas reaching the filter 22 increases, and thus much SOF adheresto the upstream tapered wall portions 53 per unit time. The amount ofthe SOF removable by oxidation per unit time at the upstream taperedwall portions 53 is limited. Therefore, if the concentration of the SOFin the exhaust gas reaching the filter 22 increases, the SOF adhering tothe upstream tapered wall portions 53 is not completely oxidized away,and then the SOF deposits thereon. Therefore, the SOF closes theupstream small holes 56.

In the filter 22, the upstream small holes 56 have sizes larger thanthose of the fine pores of the upstream tapered wall portion 53 or thepartition 54, and the partitions 54 extend generally parallel to theflow direction of the exhaust gas. However, the upstream small holes 56open to a direction perpendicular to the flow direction of the exhaustgas. Therefore, the quantity of the exhaust gas passing through theupstream holes 56 per unit surface area is larger than that passingthrough the upstream tapered wall portions 53 or the partitions 54 perunit surface area. In addition, since the upstream small holes 56 arelocated at the most upstream area of the filter 22, the particulatescontained in the exhaust gas passing through the upstream small holes 56have not been oxidized away. Therefore, the amount of the SOF passingthrough the upstream small holes 56 is larger than that passing throughthe upstream tapered wall portions 53 or the partitions 54. Thus, theSOF easily closes the upstream small holes 56.

Further, the soot has no viscosity, and thus normally does not close theupstream small holes 56, and then passes through the holes 56. On theother hand, the SOF has viscosity. Therefore, if the SOF adheres to theupstream tapered wall portions 53, the soot adheres to the SOF, and thencloses the upstream small holes 56.

Furthermore, the SOF in the exhaust gas is oxidized away by the filter22 before the SOF reaches the downstream small holes 55. Therefore, theSOF hardly closes the holes 55. However, if the particulates deposit inthe filter 22, or the concentration of the SOF in the exhaust gasdischarged from the engine increases, or the filter temperature does notrise at the engine start up to an extent that the filter demonstratesits oxidizability, the rate of oxidation of particulates by the filter22 decreases. In this case, the filter 22 does not completely oxidizethe particulates away. Therefore, much SOF reaches the downstream smallholes 55. In this case, for the same reasons as those in connection withthe upstream small holes 56, the SOF closes the downstream small holes55.

Opposed to this, in the fourth embodiment, the oxidation catalyst 22 ais arranged upstream of the filter 22. The catalyst 22 a stronglyoxidizes and removes the SOF contained in the exhaust gas(C_(m)H_(n)+O₂→C₂O+H₂O). Therefore, the amount of the SOF contained inthe exhaust gas is decreased before the exhaust gas flows into thefilter 22. If the exhaust gas flowing into the filter 22 contains almostno SOF, the SOF hardly deposits around the small holes 55,56, and hardlycloses them.

Note that, in the case that the engine is designed to inject the fuelinto the exhaust gas on the basis of the total amount of the NOx carriedby the active oxygen production agent 61 of the filter 22 in order toreduce the NOx carried by the agent 61, the concentration of the SOF inthe exhaust gas increases as explained above when fuel is injected intothe exhaust gas.

Further, note that, in the case that the engine is designed that thefuel combustion temperature in the combustion chamber becomes lower thanthe soot generation temperature by circulating the exhaust gas throughthe EGR passage by a quantity larger than that wherein the generationamount of the soot is peak, the concentration of the SOF in the exhaustgas increases as explained above when the exhaust gas is circulated intothe intake passage by a quantity larger than that wherein the generationamount of the soot is peak. In this case, the quantity of the intakeair, i.e., the oxygen decreases, and thus the fuel does not easily burnin the combustion chamber 5. Therefore, the concentration of the SOF inthe exhaust gas increases.

The fifth embodiment will be explained. In the above explained fourthembodiment, the sizes of the holes 55,56 are generally the same as eachother. Opposed to this, in the fifth embodiment, as shown in FIGS. 15Aand 15B, the sizes of the holes 55,56 are different from each other. Indetail, in the fifth embodiment, the sizes of the upstream holes 56successively increase from a central region of the filter 22 to aperipheral region thereof. The central region is the region around theaxis of the filter 22 corresponding to the axis of the housing of theturbine, i.e., of the exhaust pipe. On the other hand, the peripheralregion is the region around the central region of the filter 22 andadjacent to the periphery of the filter 22. Further, the sizes of thedownstream holes 55 successively increase from the central region of thefilter 22 to the peripheral region thereof.

In the fifth embodiment, in order to maintain the pressure loss of thefilter 22 small, the casing 23 hosing the filter 22 has a diameterlarger than that of the exhaust pipe connected thereto. Further, thecasing 23 is connected to the exhaust pipe such that the central axis ofthe casing 23 aligns with that of the exhaust pipe. Furthermore, thecasing 23 is smoothly connected to the exhaust pipe by the conicalportion of the casing 23. In this structure, the exhaust gas flowinginto the casing 23 from the exhaust pipe easily flows into the centralregion of the filter 22, and does not easily flow into the peripheralregion thereof. Therefore, the quantity of the exhaust gas flowing inthe central region of the filter 22 is larger than that flowing in theperipheral region thereof. Thus, the distribution of the exhaust gasflowing in the filter 22 is not uniform.

Opposed to this, in the fifth embodiment, the small holes 55,56 at thecentral region of the filter 22 have sizes larger than those at theperipheral region thereof. If the small holes have large sizes, theexhaust gas easily passes through the small holes. Therefore, in thefifth embodiment, the difference in quantity of the exhaust gas flowingin the central region of the filter 22 and the peripheral region thereofdecreases. That is, as explained above, the exhaust gas reaching thefilter 22 easily flows into the central region of the filter 22.However, in the fifth embodiment, the small holes 55,56 at theperipheral region of the filter 22 are enlarged. Therefore, the exhaustback pressure in the peripheral region of the filter 22 is small, andthus the exhaust gas easily flows thereinto. Therefore, the distributionof the exhaust gas is more uniform. Accordingly, the exhaust gasuniformly passes through the filter 22, and thus the filter 22 isefficiently used.

The filter 22 is heated mainly by heat derived from the exhaust gas, andfrom the chemical reaction, in the filter 22, between the specificcomponents contained in the exhaust gas. The quantity of the heat forheating the filter 22 is proportional to the quantity of the exhaust gasflowing into the filter 22. Therefore, the temperature of each portionof the filter 22 depends on the quantity of the exhaust gas flowingthereinto.

In the case that the sizes of the small holes 55,56 of the filter 22 arethe same as each other and the cross sectional areas of the exhaust gaspassages 50,51 of the filter 22 are the same as each other, the exhaustgas more easily flows into the central region of the filter 22 than intothe peripheral region thereof. Therefore, the filter 22 has atemperature at its peripheral region lower than that at its centralregion. Further, the peripheral wall surface of the filter 22 is exposedto the atmosphere having a low temperature. Therefore, the heat isdischarged from the peripheral region of the filter 22 to theatmosphere. Thus, the filter 22 has a temperature at its peripheralregion lower than that at its central region. Since an ability tooxidize the particulates at each region of the filter 22 is proportionalto the temperature thereof, the particulate oxidation ability is largearound the small holes 55,56 at the central region of the filter 22, andthus the particulates do not easily close the small holes 55,56 at thisregion. On the other hand, at the peripheral region of the filter 22,the particulate oxidation ability is small around the small holes 55,56,and thus the particulates easily close the holes 55,56 in this region.

Opposed to this, in the fifth embodiment, the sizes of the small holes55,56 successively increase from the central region of the filter 22 tothe peripheral region thereof. Therefore, the exhaust gas uniformlyflows in the filter 22, and thus the distribution of the temperature ofthe filter 22 is uniform. Accordingly, at the central region of thefilter 22, the small holes 55,56 have small sizes but the tapered wallportions 52,53 around the small holes 55,56 have high temperatures.Therefore, the particulate oxidation ability at the central region ofthe filter 22 is large, and thus the particulates do not easily closethe small holes 55,56 at this region. On the other hand, at theperipheral region of the filter 22, the tapered wall portions 52,53around the small holes 55,56 have low temperatures but the small holes55,56 have large sizes. Therefore, the particulates do not easily closethe small holes 55,56 at the peripheral region of the filter 22.

As explained above, when the amount of the particulates depositing inthe filter 22 becomes large, it is necessary to perform a control forraising the temperature of the filter 22 to a certain temperature tooxidize the depositing particulates away from the filter 22. Further,when the amount of the SO₂ carried by the filter 22 is large, it isnecessary to perform control for raising the temperature of the filter22 to a certain temperature to discharge the carried SO₂ from the filter22. However, on performing the above control, if the filter 22 has adifference in temperature, the temperature of the portion originallyhaving a low temperature may not reach a target temperature, or thetemperature of the portion originally having a high temperature mayexcessively increase beyond the target temperature. In particular, inthe case that the temperature of the portion originally having a hightemperature excessively increases beyond the target temperature, anenergy is wasted, and in some cases, the filter 22 is melted by the hightemperature.

Opposed to this, in the fifth embodiment, the difference in temperaturein the filter 22 is small. Therefore, when the temperature of the filter22 is raised to the target temperature, the temperatures of portions ofthe filter 22 are hardly excessively raised to high temperatures.Therefore, the waste of the energy and the melting of the filter 22 areavoided.

Note that, in the fifth embodiment, the sizes of the holes 55,56 mayincrease step by step, for example, in two or three steps from thecentral region of the filter 22 to its peripheral region.

Note that, since the inlet and outlet openings of the exhaust gaspassages of the catalyst 22 have sizes larger than those of the smallholes 55,56 of the filter 22, the SOF contained in the exhaust gashardly closes the openings of the exhaust gas passages of the catalyst22.

Further, a catalyst which is not a monolith type catalyst may be used asthe oxidation catalyst. Further, in place of the oxidation catalyst,there may be used a catalyst for absorbing and carrying the NOxcontained in the exhaust gas when the lean exhaust gas flows thereinto,and for releasing and reducing the carried NOx therefrom when the richexhaust gas flows thereinto even if the catalyst can remove the SOF.

The sixth embodiment will be explained. FIGS. 16A and 16B show an endview and a longitudinal cross sectional view of the filter of the sixthembodiment, respectively. In sixth embodiment, the sizes of the holes55,56 of the filter 22 are generally the same as each other. However, asshown in FIGS. 16A and 16B, the cross sectional areas of the exhaust gaspassages 50,51 successively increase from the central region of thefilter 22 to the peripheral region thereof. Accordingly, a differencebetween the quantities of the exhaust gas passing through the centraland peripheral regions of the filter 22 is small, and thus the exhaustgas uniformly flows in the filter 22.

Therefore, for the same reasons as those explained regarding the fifthembodiment, the filter 22 has a small difference in temperature, andthus the particulates do not easily close the small holes 55,56 at theperipheral region of the filter 22. Further, since the filter 22 has asmall difference in temperature, the waste of the energy and the meltingof the filter are avoided when the temperature of the filter 22 israised to the target temperature.

Note that the fifth and sixth embodiments may be combined. That is, thesizes of the small holes 55,56 of the filter 22 and the cross sectionalareas of the exhaust gas passages 50,51 may successively increase fromthe central region of the filter 22 to the peripheral region thereof.

The seventh embodiment will be explained. As shown in FIG. 17, in theseventh embodiment, the intake passage 13 is connected to an air cleaner43 upstream of the throttle valve 17. Further, in the seventhembodiment, a particulate filter (hereinafter, referred to as mainfilter) 44 is arranged in the exhaust passage downstream of the filter22. The main filter 44 is housed in a casing 45. A temperature sensor 46for detecting the temperature of the main filter 44 is attached to theupstream end of the main filter 44. The output signal of the temperaturesensor 46 is input into the input port 35 through the corresponding ADconverter 37. In the seventh embodiment, the structure of the filter 22is the same as the filter of the first embodiment. Hereinafter, thefilter 22 is referred to as the sub-filter 22.

Referring to FIGS. 18A and 18B, the main filter 44 will be explained.FIGS. 18A and 18B show an end view and a longitudinal cross sectionalview of the main filter, respectively. As shown in FIGS. 18A and 18B,the filter 44 has a honeycomb structure, and comprises a plurality ofexhaust gas passages 44 a,44 b extending parallel to each other. Thesepassages are constituted by exhaust gas inflow and outflow passages 44 aand 44 b. The inflow passages 44 a are closed at their downstream endsby plugs 44 c. On the other hand, the outflow passages 44 b are closedat their upstream ends by plugs 44 d.

The inflow and outflow passages 44 a,44 b are alternatively positioned.Thin partitions 44 e intervene between the inflow and outflow passages44 a,44 b. Four inflow passages 44 a are positioned around each outflowpassage 44 b.

In other words, one 44 a of the adjacent passages 44 a,44 b is closed atits downstream end by the plug 44 c, and the other passage 44 b isclosed at its upstream end by the plug 44 d.

The main filter 44 is formed of a porous material such as ceramics suchas cordierite containing fine pores, each having a predetermined averagesize. Therefore, as shown in FIG. 18B, the exhaust gas flows into theinflow passages 44 a, and then into the adjacent outflow passages 44 bthrough the fine pores of the surrounding partitions 44 e. When theexhaust gas flows in the passages 44 a,44 b, the particulates containedin the exhaust gas are collected by the wall surfaces of the partitions44 e defining the passages 44 a,44 b. Further, when the exhaust gaspasses through the fine pores of the partitions 44 e, the particulatescontained in the exhaust gas are collected by the wall surfaces definingthe fine pores.

Note that, similar to the sub-filter 22, the main filter 44 also carriesprecious metal catalyst and active oxygen production agent therein.Further, similar to the sub-filter 22, the end openings of the exhaustgas passages 44 a,44 b of the main filter 44 may be partially closed bytapered wall portions and have small holes at the tips of the taperedwall portions. Furthermore, the end openings of the exhaust gas passages44 a,44 b of the main filter 44 may be completely closed by tapered wallportions and have no small hole at the tips of the tapered wallportions. Further, the main filter 44 may be of a monolith type suchthat the end openings of the exhaust gas passages 44 a 44 b are notclosed, and thus are completely open.

As explained in connection with the first embodiment, the sub-filter 22has a pressure loss smaller than that of the main filter 44. Therefore,although the sub-filter 22 is arranged upstream of the main filter 44,the total pressure loss of the exhaust gas purification device does notlargely increase. Of course, the total particulate collection ratio ofthe exhaust gas purification device does not largely decrease.

As explained above, the active oxygen production agent of the mainfilter 44 carries the oxygen in the form of the nitrate ions when theatmosphere surrounding the agent is oxidative. That is, the agent of themain filter 44 serves as a NOx carrier agent for carrying the NOxtherein when the atmosphere surrounding the agent is oxidative. Theamount of the NOx which the agent can carry therein has an upper limit.If the amount of the NOx reaches the upper limit, the agent of the mainfilter 44 does not newly carry the NOx, and then the NOx flows out ofthe main filter 44. Therefore, it is necessary to purify the NOx carriedin the agent by reducing the same before the amount of the carried NOxreaches the upper limit.

The active oxygen production agent releases the oxygen carried in theform of the nitrate ions when the atmosphere surrounding the agentbecomes reductive. In other words, the agent releases the NOx carried inthe form of the nitrate ions when the atmosphere surrounding the agentbecomes reductive. At this time, as shown in FIG. 19C, the NOx releasedfrom the agent is reduced by the hydrocarbon and the carbon monoxidecontained in the exhaust gas.

In the seventh embodiment, before the amount of the carried NOx reachesan allowed upper limit, the NOx carried in the agent of the main filter44 is reduced and purified by supplying the rich or generallystoichiometric exhaust gas to the filter 44. Note that, in considerationof this function of the filter 44, the filter 44 has a NOx catalystcomprising the NOx carrier agent and the precious metal catalyst.

Note that if the rich exhaust gas having a low concentration of theoxygen is supplied to the main filter 44, the ratio of the hydrocarbonand the carbon monoxide oxidized away by the main filter 44 is small. Onthe other hand, if the lean or generally stoichiometric exhaust gas issupplied to the main filter 44, the ratio of the hydrocarbon and thecarbon monoxide oxidized away by the filter 44 is large. Further, FIGS.19A and 19B correspond to FIGS. 6A and 6B, respectively.

As explained above, the filters 22,44 purify four kinds of componentssuch as the particulates, the NOx, the carbon monoxide, and thehydrocarbon, depending on the characteristics of the exhaust gas flowingthereinto. The platinum and the active oxygen production agent carriedby the filters 22,44 are more active when the temperatures thereof arehigh. Therefore, the purification ratios of the above four kinds of thecomponents by the filters 22,44 depend on the temperature of the exhaustgas flowing thereinto, and become large when an exhaust gas having ahigh temperature flows thereinto.

In the seventh embodiment, the sub-filter 22 is arranged directlydownstream of the exhaust manifold 17. Therefore, the hot exhaust gasimmediately after being discharged from the combustion chamber 5 flowsinto the sub-filter 22, and thus the temperature of the sub-filter 22 ismaintained high. Of course, at the engine start up, the exhaust gasdischarged from the combustion chamber 5 has a low temperature but,according to the seventh embodiment, the temperature of the sub-filter22 is rapidly raised by the exhaust gas and is maintained high.Accordingly, in the seventh embodiment, the purification ratios of theabove four kinds of components are high.

The sub-filter 22 also has an ability to oxidize the particulates, andthus the decreased amount of the particulates flows into the main filter44. Therefore, almost all of the particulates contained in the exhaustgas may be oxidized away by the main filter 44. Otherwise, as the mainfilter 44 is required to oxidize small amount of the particulates, it ispermitted to downscale the filter 44. Note that, in order to maintainthe temperature of the sub-filter 22 higher than a certain temperature,the sub-filter 22 should be arranged near the exhaust port 10.Therefore, as shown in FIG. 20, the sub-filter 22 may be arranged in thebranch pipe of the manifold 17.

The eighth embodiment will be explained. In the eighth embodiment, asshown in FIG. 21, a bypass passage 48 which bypasses the main filter 44extends from the exhaust pipe 47 between the sub- and main filters22,44. The passage 48 at its upstream end is connected to the exhaustpipe 47 upstream of the main filter 44, and at its downstream end isconnected to the exhaust pipe 47 downstream of the main filter 44. Thebypass passage 48 and the main filter 44 are in parallel with eachother. In the eighth embodiment, a switch valve 49 is arranged at theconnection of the bypass passage 48 and the exhaust pipe 47 upstream ofthe main filter 44. The valve 49 serves to switch the flow of theexhaust gas to the main filter 44 and the bypass passage 48.

The active oxygen production agent carries the SOx contained in theexhaust gas when the lean exhaust gas flows thereinto. If the amount ofthe SOx carried by the agent increases, the amount of the NOx which theagent can carry decreases. Therefore, before the amount of the NOx whichthe agent can carry decreases under an allowed lower limit, it isnecessary to release the SOx from the agent.

In the eighth embodiment, the sub-filter 22 is arranged upstream of themain filter 44. The active oxygen production agent of the sub-filter 22carries the SOx contained in the exhaust gas when the lean exhaust gasflows thereinto, and thus the exhaust gas flowing into the maim filter44 contains a very small amount of the SOx. Therefore, in the eighthembodiment, it is hardly necessary to release the SOx from the activeoxygen production agent of the main filter 44.

The active oxygen production agent of the sub-filter 22 has an allowedupper limit to carry the SOx. Therefore, before the amount of the SOxcarried by the agent of the sub-filter 22 reaches the allowed upperlimit, it is necessary to release the SOx from the agent of thesub-filter 22. Next, referring to FIGS. 22A and 22B, the release of theSOx from the agent of the sub-filter 22 will be explained.

FIG. 22A shows a relationship between the temperature T of the activeoxygen production agent when the atmosphere surrounding the agent isrich or generally stoichiometric, and the NOx releasing ratio f(T) andthe SOx releasing ratio g(T) from the agent. FIG. 22B shows the periodthat the atmosphere surrounding the agent is maintained rich orgenerally stoichiometric, and the total NOx releasing amount and thetotal SOx releasing amount from the agent when the agent has atemperature lower the temperature To shown in FIG. 22A.

As can be understood from the FIG. 22A, when the agent temperature T islower than the temperature To and the atmosphere surrounding the agentis rich or generally stoichiometric, the agent releases the NOx buthardly releases the SOx therefrom. Therefore, in state that the agenttemperature T is lower than the temperature To, the agent releases avery small amount of the SOx even if the atmosphere surrounding theagent is maintained rich or stoichiometric for a long time.

In the case that the engine is a compression ignition type engine, themain filter 44 often has a temperature lower than the temperature To.Therefore, even if the rich or generally stoichiometric exhaust gasflows into the main filter 44, the active oxygen production agent of themain filter 44 hardly releases the SOx therefrom.

In the eighth embodiment, when it is necessary to release the SOx fromthe active oxygen production agent of the sub-filter 22, the temperatureof the sub-filter 22 is raised above the temperature To and the richexhaust gas is supplied to the sub-filter 22, or the temperature of theexhaust gas flowing into the sub-filter 22 is raised above a temperaturewhereat the active oxygen production agent of the sub-filter 22 releasesthe SOx therefrom and the rich exhaust gas is supplied.

According to this, the sub-filter 22 releases the SOx therefrom. Inconsideration of this function of the sub-filter 22, the sub-filter 22serves as a SOx carrying agent for carrying the SOx contained in theexhaust gas when the lean exhaust gas flows into the sub-filter 22.Further, since the sub-filter 22 is arranged near the engine body, thetemperature of the sub-filter 22 is easily raised.

In the eighth embodiment, when the sub-filter 22 releases the SOxtherefrom, the switch valve 47 is positioned as shown by the chain lineof the FIG. 21 such that the exhaust gas flows into the bypass passages48. According to this, the SOx released from the sub-filter 22 hardlyflows into the main filter 44. On the other hand, when the sub-filter 22does not release the SOx therefrom, the switch valve 47 is positioned asshown by the solid line of the FIG. 21.

Note that the sub-filter 22 carries the NOx contained in the exhaust gaswhen the lean exhaust gas flows thereinto, and releases the carried NOxtherefrom when the rich or generally stoichiometric exhaust gas flowsthereinto, and reduces and removes the released NOx by hydrocarbon andcarbon monoxide contained in the exhaust gas. Therefore, when the richor generally stoichiometric exhaust gas is supplied to the sub-filter 22to release the SOx from the sub-filter 22, the sub-filter 22 releasesthe NOx therefrom, and the released NOx is reduced and removed by thehydrocarbon and the monoxide contained in the exhaust gas. Therefore,even if the exhaust gas bypasses the main filter 44, the NOx, thehydrocarbon and the carbon monoxide hardly flow out of the exhaust gaspurification device.

Note that, in order to stop releasing the SOx from the sub-filter 22,the air fuel ratio of the exhaust gas is changed from a rich orgenerally stoichiometric air fuel ratio to a lean air fuel ratio, andthe position of the switch valve 47 is changed to a position shown inthe solid line of the FIG. 21.

The active oxygen production agent easily releases the SOx therefrom ifthe agent carries the SOx in the form of the sulfate ions or theunstable sulfate. In the eighth embodiment, as the agent of thesub-filter 22, an active oxygen production agent carrying at least oneof transition metal such as copper, iron, manganese and nickel, sodium,titanium and lithium on an alumina carrier is used. Therefore, in theeighth embodiment, the agent of the sub-filter 22 easily releases theSOx therefrom.

In the eighth embodiment, in order to release the NOx from the mainfilter 44, the air fuel ratio of the exhaust gas is changed from thelean air fuel ratio to the rich or generally stoichiometric air fuelratio. However, the period for maintaining the air fuel ratio of theexhaust gas lean or generally stoichiometric is short. Therefore, thetemperature of the sub-filter 22 hardly rises above the temperature To.Thus, the sub-filter 22 does not release the SOx therefrom, and no SOxflows into the main filter 44. Of course, in the state that the mainfilter 44 has a temperature lower than the temperature To, if theexhaust gas flowing into the main filter 44 has a rich air fuel ratio,the main filter 44 releases the NOx therefrom, and the NOx is reducedand removed.

The ninth embodiment will be explained. When the air fuel ratio of theexhaust gas is made rich or generally stoichiometric to release the SOxfrom the sub-filter 22, some hydrocarbon or carbon monoxide flows out ofthe sub-filter 22. The hydrocarbon and carbon monoxide should bepurified. According to the ninth embodiment, as shown in FIG. 23, in theexhaust gas purification device of the eighth embodiment, a three waycatalyst 44 a is arranged in the bypass passage 48. The three waycatalyst 44 a oxidizes the hydrocarbon and the carbon monoxide when therich or generally stoichiometric exhaust gas flows thereinto.

According to the ninth embodiment, when the sub-filter 22 releases theSOx therefrom, the switch valve 47 is positioned at a position shown bythe chain line of the FIG. 23, and thus the hydrocarbon and the carbonmonoxide flow from the sub-filter 22 into the bypass passage 48.Therefore, the hydrocarbon and the carbon monoxide are oxidized andpurified by the three way catalyst. Note that it is sufficient that thecatalyst arranged in the bypass passage 48 has an ability to oxidize thecomponents such as hydrocarbon and carbon monoxide away. Therefore, anoxidation catalyst may be used in place of the three way catalyst.

The tenth embodiment will be explained. In the tenth embodiment, inplace of the filter of the first embodiment, a exhaust gas purificationcatalyst 22 carrying a hydrocarbon collection agent is arranged in theexhaust passage of the engine. The catalyst 22 comprises the samestructure as that of the filter of the first embodiment. Unburnedhydrocarbon contained in the exhaust gas adheres to the hydrocarboncollection agent of the catalyst 22. In other words, the hydrocarboncollection agent collects unburned hydrocarbon contained in the exhaustgas. In the tenth embodiment, as the hydrocarbon collection agent,layers formed of, for example, alumina are entirely formed on both sidewall surfaces of the partitions 54, the wall surfaces defining the finepores of the partitions 54, and both side wall surfaces of the taperedwall portions 52,53.

The hydrocarbon collection agent collects the unburned hydrocarboncontained in the exhaust gas by the adhering of the hydrocarbon theretowhen the agent has a temperature lower than the temperature at which thehydrocarbon leaves the agent. On the other hand, the hydrocarboncollection agent releases the collected hydrocarbon therefrom when theagent has a temperature higher than the temperature at which thehydrocarbon leaves the agent. The temperature at which the hydrocarbonleaves the hydrocarbon collection agent is set such that the agent doesnot release the hydrocarbon when a later explained hydrocarbonpurification catalyst has a temperature lower than the temperature atwhich the catalyst does not oxidize and purify the unburned hydrocarbon.

The exhaust gas purification catalyst 22 has a hydrocarbon purificationcatalyst for oxidizing and purifying the unburned hydrocarbon. In thetenth embodiment, as the hydrocarbon purification catalyst, preciousmetal catalyst such as platinum is carried on the alumina carrying layerof the catalyst 22. The hydrocarbon purification catalyst oxidizes andpurifies the unburned hydrocarbon when its temperature is higher than ahydrocarbon purification temperature.

In the tenth embodiment, the pressure loss of the exhaust gaspurification catalyst 22 and the unburned hydrocarbon and particulatecollection ratios of the catalyst 22 can be adjusted by adjusting thesizes of the small holes 55,56 of the catalyst 22.

The action of the exhaust gas purification catalyst of the tenthembodiment will be explained. Even in the state that the hydrocarboncollection agent has a low temperature, if the quantity of the exhaustgas flowing into the catalyst 22 per unit time rapidly increases, thehydrocarbon collected by the agent may be released from the wallsurfaces or fine pores of the partitions 54 by the exhaust gas. At thistime, the hydrocarbon purification catalyst also has a low temperature,and thus does not purify the hydrocarbon. In this case, if the exhaustgas passages of the catalyst 22 completely open at the outlets thereof,the hydrocarbon flows out of the catalyst 22.

When the catalyst 22 has a low temperature, for example, at engine startup, the hydrocarbon purification catalyst also has a temperature lowerthan the hydrocarbon purification temperature, and thus the unburnedhydrocarbon contained in the exhaust gas is not oxidized and purified bythe hydrocarbon purification catalyst. However, at this time, thehydrocarbon collection agent has a temperature lower than thehydrocarbon release temperature, and thus the unburned hydrocarboncontained in the exhaust gas is collected by the hydrocarbon collectionagent. Therefore, the unburned hydrocarbon hardly flows out of thecatalyst 22.

On the other hand, when the temperature of the exhaust gas dischargedfrom the engine successively rises and the temperature of thehydrocarbon collection agent exceeds the hydrocarbon releasetemperature, the unburned hydrocarbon leaves the hydrocarbon collectionagent. At this time, the hydrocarbon purification catalyst has atemperature higher than the hydrocarbon purification temperature.Therefore, the unburned hydrocarbon leaving the hydrocarbon collectionagent is oxidized and purified by the hydrocarbon purification catalyst.Thus, the unburned hydrocarbon hardly flows out of the catalyst 22.

In the state that the hydrocarbon collection agent has a temperaturelower than the hydrocarbon release temperature, if the quantity of theexhaust gas passing through the fine pores of the partitions 54 of thecatalyst 22 rapidly increases, the unburned hydrocarbon may leave thefine pores of the partitions 54. At this time, the hydrocarbonpurification catalyst has a temperature lower than the hydrocarbonpurification temperature, and thus the leaving hydrocarbon is hardlypurified by the hydrocarbon purification catalyst.

In the tenth embodiment, the upstream openings of the outflow passages51 of the catalyst 22 are partially closed by the upstream tapered wallportions 53, and thus almost all exhaust gas flows into the inflowpassages 50 of the catalyst 22. Further, the downstream openings of theinflow passages 50 of the catalyst 22 are partially closed by thedownstream tapered wall portions 52, and thus almost all exhaust gaspasses through the fine pores of the partitions 54. Therefore, theparticulates are collected in the fine pores of the partitions 54. Ofcourse, the particulates are collected on the wall surfaces of thepartitions 54 defining the inflow passages 50.

In the state that the particulates are collected and deposit in the finepores and on the wall surfaces of the partitions 54, the exhaust gasdoes not easily pass through the fine pores of the partitions 54. As aresult, even if the quantity of the exhaust gas flowing into thecatalyst 22 per unit time rapidly increases, the quantity of the exhaustgas passing through the fine pores of the partitions 54 per unit timedoes not largely increase. Therefore, the unburned hydrocarbon hardlyleaves the fine pores of the partitions 54. Thus, the unburnedhydrocarbon hardly flows out of the catalyst 22.

Note that, similar to the filter of the first embodiment, the catalyst22 of the tenth embodiment has an active oxygen production agent, andthus successively oxidizes the particulates away for a short time.Therefore, the amount of the particulates collected in the fine pores ofthe partitions 54 and on the wall surfaces of the partitions 54 definingthe inflow passages 50 is maintained small.

As explained above, the concentration of the oxygen around the activeoxygen production agent decreases when the particulates adhere to theagent even in the state that the atmosphere surrounding the agent 61 islean. Further, other than this case, the concentration of the oxygenaround the active oxygen production agent decreases when the richexhaust gas flows into the exhaust gas purification catalyst 22, andthus the atmosphere surrounding the agent becomes rich.

As explained above, in the state that the atmosphere surrounding theactive oxygen production agent is lean, when the particulates adhere tothe agent to decrease the concentration of the oxygen therearound, theNOx leaves the agent. In this case, the leaving NOx is carried again bythe active oxygen production agent. On the other hand, as explainedabove, when the rich exhaust gas flows into the catalyst 22 to make theatmosphere surrounding the active oxygen production agent rich, the NOxleaves the agent. In this case, the leaving NOx is reduced and purifiedby the unburned hydrocarbon contained in the exhaust gas together withthe action of the platinum. That is, if the engine operation iscontrolled to discharge the rich exhaust gas therefrom, the NOx carriedby the active oxygen production agent is reduced and purified.Therefore, the catalyst 22 of the tenth embodiment has a NOx catalystcomprising the active oxygen production agent and the platinum.

Note that, as explained above, in the case that the catalyst 22 has anactive oxygen production agent, the catalyst 22 oxidizes theparticulates collected therein away even if the catalyst 22 has a lowtemperature. However, if the catalyst 22 has a lower temperature, theparticulates successively deposit in the catalyst 22. As explainedabove, the inlets of the inflow passages 50 and the outlets of theoutflow passages 51 of the catalyst 22 are defined by the wall surfacesof the tapered wall portions 52,53, and thus the exhaust gas does notflow with turbulence at the inlets of the inflow passages 50 and theoutlets of the outflow passages 51. Therefore, the catalyst 22potentially has a low pressure loss. Thus, even if the particulatesdeposit in the catalyst 22, the pressure loss of the catalyst 22 ismaintained low.

Of course, if the amount of the unburned hydrocarbon collected by thehydrocarbon collection agent or the amount of the particulatesdepositing in the catalyst 22 increases, the ability of the collectionof the unburned hydrocarbon by the hydrocarbon collection agentdecreases. However, for example, after the engine starts up, thetemperature of the catalyst 22 rises, and thus the unburned hydrocarbonand the particulates collected in the catalyst 22 are oxidized.Therefore, at the next engine start up, the very small amount of theunburned hydrocarbon and the particulates deposit in the catalyst 22.Thus, the unburned hydrocarbon leaving the catalyst 22 immediately afterthe engine start up is assuredly collected by the hydrocarbon collectionagent of the catalyst 22.

In the tenth embodiment, almost all exhaust gas flows into the inflowpassages 50 of the catalyst 22, passes through the fine pores of thepartitions 54, and flows into the outflow passages 51. When the exhaustgas passes through the fine pores of the partitions 54, someparticulates are collected in the fine pores of the partitions 54. Ifthe particulates are collected in the fine pores of the partitions 54,the exhaust gas does not easily pass through the partitions 54.Therefore, even if the quantity of the exhaust gas flowing into thecatalyst 22 rapidly increases, the quantity of the exhaust gas passingthrough the fine pores of the partitions 54 does not largely increase.Thus, the unburned hydrocarbon hardly leaves the fine pores of thepartitions 54.

Therefore, in the state that the hydrocarbon collection agent has atemperature lower than the hydrocarbon release temperature and thehydrocarbon purification catalyst has a temperature lower than thehydrocarbon purification temperature, the unburned hydrocarbon hardlyleaves the fine pores of the partitions 54 even if the quantity of theexhaust gas flowing into the catalyst 22 rapidly increases. That is, inthe tenth embodiment, in the state that the hydrocarbon purificationcatalyst does not purify the unburned hydrocarbon, even if the quantityof the exhaust gas flowing into the catalyst 22 per unit time rapidlyincreases, almost all unburned hydrocarbon remains on the hydrocarboncollection agent.

The second method for producing a particulate filter of the inventionwill be explained. The method explained below is a method for producinga particulate filter 22 shown in FIGS. 24A and 24B. The filter 22 shownin FIGS. 24A and 24B is the same as that shown in FIGS. 1A and 1B exceptthat each tapered wall portion of the filter shown in FIGS. 24A and 24Bhave a quadrangular pyramid shape while each tapered wall portion of thefilter shown in FIGS. 1A and 1B have a conical shape.

According to the second method, first, as shown in FIGS. 25A and 25B, asubstrate 100 formed of porous material such as cordierite and having ahoneycomb structure is prepared. The substrate 100 has exhaust gaspassages 50,51 defined by partitions 54. The partitions 54 form agridiron pattern.

Next, as shown in FIG. 25B, a closure device 101 for partially closingend openings of the exhaust gas passages 50,51 is pressed onto one ofthe end faces of the substrate 100. FIGS. 26A and 26B show the closuredevice 101 in detail. Referring to FIG. 26A showing a plan view of theclosure device 101, the device 101 has predetermined numbers ofprojections 102. As can be understood from FIG. 26B showing one of theprojections 102, each projection 102 has substantially a regularquadrangular pyramid shape. The projections 102 are arranged in apattern that four adjacent ridges 103 of four adjacent projections 102converge. Further, a pin 104 is arranged at each area where fouradjacent ridges 103 of four adjacent projections 102 converge.

The closure device 101 is pressed onto one of the end faces of thesubstrate 100 such that each projection 102 is inserted into thecorresponding exhaust gas passage 50. When the device 101 is pressedonto one end face of the substrate 100, four end portions of fouradjacent partitions 54 defining each exhaust gas passage 51 are gatheredtoward each other by corresponding four adjacent projections 102. Eachpin 104 of the closure device 101 exists in each area enclosed by fourgathered end portions of four adjacent partitions 54. As a result, fourend portions of four adjacent partitions 54 defining each exhaust gaspassage 51 are partially connected to each other while a small hole 56is formed therein by the pin 104. Thus, upstream tapered wall portions53 having the small holes 56 are formed.

Next, the closure device 101 is pressed onto the other end face of thesubstrate 100 such that each projection 102 is inserted into eachexhaust gas passage 51. Thus, downstream tapered wall portions 52 havingsmall holes 55 are formed.

As explained above, according to the second method, the closure of theend openings of the exhaust gas passages, i.e., the formation of thetapered wall portions for closing the end openings of the exhaust gaspassages, and the formation of the small holes in the tapered wallportions are performed at a time.

The third method for producing a filter will be explained. The closuredevice 101 used in the third method comprises an opening closure device105 shown in FIG. 27A and a hole formation device 106 shown in FIG. 28A.

Referring to FIG. 27A showing a plan view of the opening closure device105, the device 105 has predetermined numbers of projections 102. FIG.27B shows one of the projections 102. As can be understood from FIG.27B, each projection 102 has substantially a regular quadrangularpyramid shape. Each projection 102 is arranged in a pattern that fouradjacent ridges 103 of four adjacent projections 102 converge.

On the other hand, referring to FIG. 28A showing a plan view of the holeformation device 106, the device 106 has predetermined numbers of pins104. FIG. 28B shows four pins 104. Each pin 104 is arranged at each areawhere four adjacent ridges 103 of four adjacent projections 102converge.

According to the third method, as shown in FIG. 29A, first, the openingclosure device 105 is pressed onto one of the end faces of the substrate100 such that each projection 102 is inserted into each exhaust gaspassage 50. When the device 105 is pressed onto one end face of thesubstrate 100, four end portions of four adjacent partitions 54 definingeach exhaust gas passage 51 are gathered toward each other bycorresponding four adjacent projections 102. Thus, four end portions offour adjacent partitions 54 defining each exhaust gas passage 51 areconnected to each other to completely close the end opening of eachexhaust gas passage 51 by a corresponding tapered wall portion.

Next, as shown in FIG. 29B, the hole formation device 106 is pressedonto one end face of the substrate 100 such that each pin 104 piercesthe tip of the corresponding tapered wall portion which completelycloses the end opening of the corresponding exhaust gas passage 51. As aresult, a small holes 56 is formed in the tip of each tapered wallportion.

Regarding the other end face of the substrate 100, the similar processesare performed. That is, the opening closure device 105 is pressed ontothe other end face of the substrate 100 such that each projection 102 isinserted into the corresponding exhaust gas passage 51. As a result, theend opening of each exhaust gas passage 50 is completely closed by acorresponding tapered wall portion. Next, the hole formation device 106is pressed onto the other end face of the substrate 100 such that eachpin 104 pierces the tip of the corresponding tapered wall portion whichcompletely closes the end opening of the corresponding exhaust gaspassage 50. As a result, a small hole 55 is formed in the tip of eachtapered wall portion.

According to the third method, first, the closure of the end openings ofthe exhaust gas passages, i.e., the formation of the tapered wallportions for closing the end openings of the exhaust gas passages isperformed, and thereafter the formation of the small holes in thetapered wall portions is performed. Of course, in the third method, thefollowing may be employed. That is, first, the end openings of theexhaust gas passages 50 are completely closed by the tapered wallportions, and then the end openings of the exhaust gas passages 51 arecompletely closed by the tapered wall portions, and then the small holesare formed in the tapered wall portions.

The fourth method for producing a filter will be explained. As shown inFIG. 30, the closure device 101 used in the fourth method comprises anopening closure device 107 and a hole formation device 108.

Similar to the opening closure device 105 as shown in FIG. 29A, theopening closure device 107 has predetermined numbers of projections 102.Similar to the device 105, each projection 102 of the device 107 hassubstantially a regular quadrangular pyramid shape, and the projections102 are arranged in a pattern that four adjacent ridges 103 of fouradjacent projections 102 converge. Unlike the device 105, the device 107has through holes 109. Each hole 109 is positioned at each area wherefour adjacent ridges 103 of four adjacent projections 102 converge.

On the other hand, similar to the hole formation device 106 shown inFIGS. 28A and 28B, the hole formation device 108 has predeterminednumbers of pins 104. The pins 104 are arranged in a pattern that eachpin 104 is inserted into the corresponding through hole 109.

According to the fourth method, similar to the third method, first, theopening closure device 107 is pressed onto one of the end faces of thesubstrate 100 such that each projection 102 is inserted into thecorresponding exhaust gas passage 50. As a result, the end opening ofeach exhaust gas passage 51 is completely closed by a correspondingtapered wall portion.

Next, in the state that the device 107 is pressed onto one end face ofthe substrate 100, the hole formation device 108 is pressed onto thedevice 107 such that each pin 104 is inserted into the correspondingthrough hole 109. As a result, each pin 104 pierces the tip of thecorresponding tapered wall portion which completely closes the endopening of the corresponding exhaust gas passage 51. As a result, asmall hole 56 is formed in the tip of each tapered wall portion.

Regarding the other end face of the substrate 100, the similar processesare performed. That is, similar to the third method, the opening closuredevice 107 is pressed onto the other end face of the substrate 100 suchthat each projection 102 is inserted into the corresponding exhaust gaspassage 51. As a result, the end opening of each exhaust gas passage 50is completely closed by a corresponding tapered wall portion. Next, inthe state that the device 107 is pressed onto the other end face of thesubstrate 100, the hole formation device 108 is pressed onto the device107 such that each pin 104 is inserted into the corresponding throughhole 109. As a result, each pin 104 pierces the tip of the correspondingtapered wall portion which completely closes the end opening of thecorresponding exhaust gas passage 50. Thus, a small hole 55 is formed inthe tip of each tapered wall portion.

According to the fourth method, similar to the third method, first, theclosure of the end openings of the exhaust gas passages, i.e., theformation of the tapered wall portions for closing the end openings ofthe exhaust gas passages is performed, and then the formation of thesmall holes in the tapered wall portions is performed.

According to the fourth method, in the state that the tapered wallportions of the substrate 100 are pressed by the device 107, each smallhole is formed in the corresponding tapered wall portion by the device108. Therefore, when each pin 104 of the device 108 is pressed onto thecorresponding tapered wall portion, the tapered wall portions are hardlysubject to damage.

Further, in the above explained third method, after the tapered wallportions are formed, the small holes are formed by pressing the holeformation device 106 onto the end face of the substrate 100. Therefore,before the device 106 is pressed onto the end face of the substrate, itis necessary to exactly position the device 106 such that each pin 104of the device 106 corresponds to the tip of the corresponding taperedwall portion. This is burdensome. Opposed to this, according to thefourth method, each small hole is formed in the corresponding taperedwall portion simply by inserting each pin 104 of the device 108 into thecorresponding through hole 109 of the device 107. Therefore, it is notnecessary to perform a process to position the device 108 such that eachpin 104 of the device 108 corresponds to the tip of the correspondingtapered wall portion to form small holes therein by the device 108.

The fifth method for producing a filter will be explained. The closuredevice 101 used in the fifth method comprises the opening closure device105 shown in FIGS. 27A and 27B and a hole formation device 110 shown inthe plan view of FIG. 31. FIGS. 32A and 32B show the device 110 indetail.

FIG. 32A shows a plan view of the hole formation device 110 in which anend wall 113 is omitted. As can be understood from FIG. 32A, the device110 has predetermined numbers of drill members 112. As can be understoodfrom FIG. 32B showing one of the drill members 112, each member 112 hasa gear 113 and a drill 114 which extends from the central portion of thegear 113 in a direction perpendicular to the end wall surface of thegear 113.

As shown in FIG. 32A, each drill member 112 engages with correspondingintermediate gears 115. Two adjacent drill members 112 are connected viaone intermediate gear 115. A certain drill member 112 engages with adrive gear 116. The gear 116 is rotated by a suitable drive means suchas an electric motor. When the drive gear 116 is rotated, the drillmember 112 engaging with the drive gear 116 is rotated, and then therotation of the member 112 is transmitted to all remaining drill members112 via the intermediate gears 116. As a result, each drill member 112is rotated about its longitudinal axis.

Note that the drills 114 of the drill members 112 are projected from theend wall 113 of the hole forming device 110. The drills 114 are arrangedin the same pattern as that regarding the pins 104 of the device 106shown in FIGS. 28A and 28B.

According to the fifth method, similar to the third method, the openingclosure device 105 shown in FIG. 27A is pressed onto one of the endfaces of the substrate 100 to completely close the end opening of eachexhaust gas passages 50 by a tapered wall portion. Next, the drive gear116 of the hole formation device 110 is rotated, and the device 110 ispressed onto one end face of the substrate 100 such that the drill 114of each drill member 112 pierces the tip of the corresponding taperedwall portion which completely closes the end opening of thecorresponding exhaust gas passage 51. As a result, a small hole 56 isformed in the tip of each tapered wall portion by the correspondingrotating drill 114.

Regarding the other end face of the substrate 100, the similar processesare performed. That is, similar to the third method, the opening closuredevice 105 is pressed onto the other end face of the substrate 100 tocompletely close the end opening of each exhaust gas passage 51 by thecorresponding tapered wall portion. Next, the drive gear 116 of the holeformation device 110 is rotated, and the device 110 is pressed onto theother end face of the substrate 100 such that the drill 114 of eachdrill member 112 pierces the tip of the corresponding tapered wallportion which completely closes the end opening of the correspondingexhaust gas passage 50. As a result, a small hole 55 is formed in thetip of each tapered wall portion by the corresponding rotating drill114.

According to the fifth method, the small holes are formed in the tips ofthe tapered wall portions by the rotating drills 114. Therefore, thetapered wall portions are hardly subject to damage when the small holesare formed, comparing with the case that the holes are formed in thetips of the tapered wall portions simply by pins.

The sixth method for producing a filter will be explained. The closuredevice 101 used in the sixth method comprises the opening closure device105 shown in FIG. 27A and a hole formation device 117 shown in FIG. 33A.In FIG. 33A, the end wall 118 of the device 117 shown in FIG. 33B isomitted.

As shown in FIG. 33A, the device 117 has predetermined numbers of drillmembers 119. As shown in FIG. 33B, each drill member 119 has a ball 120and a drill 121 which extends from the ball 120.

As shown in FIG. 33B, a plurality of annular grooves 123 are formed in adisc body 122 of the device 117. The center of each groove 123corresponds to the center of the body 122. The body 122 is rotated by asuitable means such as an electric motor about an axis A shown in FIG.33B.

The ball 120 of each drill member 119 is housed in the correspondinggroove 123 such that the ball 120 is in contact with the side wallsurface defining the groove 123. The drills 121 of the drill members 119project from the end wall 118 of the device 117. The drills 121 arearranged in the same pattern as that of the pins 104 of the device 106shown in FIG. 28A.

When the body 122 is rotated, the ball 120 of each drill member 119 isrotated by the side wall surface of the corresponding groove 123 of thebody 122. As a result, the drill 121 of each drill member 119 is rotatedabout its longitudinal axis.

Note that each drill member 119 may have a bevel gear in place of theball 120. In this case, a bevel gear is provided on the side wallsurface of each groove 123. The bevel gear of each drill member 119engages with the bevel gear of the side wall surface of thecorresponding groove 123. When the body 122 is rotated, the bevel gearof each drill member 119 is rotated by the body 122.

According to the sixth method, similar to the third method, the openingclosure device 105 shown in FIG. 27A is pressed onto one of the endfaces of the substrate 100 to completely close the end openings of theexhaust gas passages 51 by the tapered wall portions. Next, the body 122of the device 117 is rotated, and the device 117 is pressed onto one endface of the substrate 100 such that the drill 121 of each drill member119 pierces the tip of the corresponding tapered wall portion whichcompletely closes the end opening of the corresponding exhaust gaspassage 51. As a result, a small hole 56 is formed in the tip of eachtapered wall portion by the corresponding rotating drill 121.

Regarding the other end face of the substrate 100, the similar processesare performed. That is, similar to the third method, the opening closuredevice 105 is pressed onto the other end face of the substrate 100 tocompletely close the end openings of the exhaust gas passages 50 by thetapered wall portions. Next, the body 122 of the device 117 is rotated,and the device 117 is pressed onto the other end face of the substrate100 such that the drill 121 of each drill member 119 pierces the tip ofthe corresponding tapered wall portion which completely closes the endopening of the corresponding exhaust gas passage 50. As a result, asmall hole 55 is formed in the tip of each tapered wall portion by thecorresponding rotating drill 121.

The seventh method for producing a filter will be explained. The closuredevice 101 used in the seventh method comprises the opening closuredevice 105 shown in FIG. 27A and a hole formation device 124 shown inFIG. 34B.

The device 124 is a disc shape having generally the same diameter asthat of the device 105. Further, the device 124 has a body 125 and ashaving layer 126 attached to the body 125. The layer 126 is formed ofabrasive for shaving the tips of the tapered wall portions of thesubstrate 100. Furthermore, the device 124 is rotated about an axis B bya suitable means such as an electric motor.

According to the seventh method, similar to the third method, as shownin FIG. 34A, the device 105 shown in FIG. 27A is pressed onto one of theend faces of the substrate 100 to completely close the end openings ofthe exhaust gas passages 51 by the tapered wall portions. Next, thedevice 124 is rotated about the axis B, and is pressed onto one end faceof the substrate 100. As a result, the shaving layer 126 of the device124 is pressed onto the tips of the tapered wall portions of thesubstrate 100. The tips of the tapered wall portions are shaved by theshaving layer 126. As a result, a small hole 56 is formed in the tip ofeach tapered wall portion.

Regarding the other end face of the substrate 100, the similar processesare performed. That is, similar to the third method, the device 105 ispressed onto the other end face of the substrate 100 to completely closethe end openings of the exhaust gas passages 50 by the tapered wallportions. Next, the device 124 is rotated about the axis B, and pressedonto the other end face of the substrate 100. As a result, a small hole55 is formed in the tip of each tapered wall portion.

The above explained methods have an advantage that small holes 55,56each having a desired opening area, i.e., the generally same openingarea can be obtained. The opening area of each small hole 55,56influences the pressure loss and the particulate collection ratio of thefilter 22. That is, the pressure loss and the particulate collectionratio of the filter 22 are changed by changing the opening area of theholes 55,56. According to the above explained methods, holes 55,56 eachhaving a desired opening area can be obtained, and thus the filter 22having the desired pressure loss and the desired particulate collectionratio is obtained.

While the invention has been described by reference to specificembodiments chosen for purposes of illustration, it should be apparentthat numerous modifications could be made thereto by those skilled inthe art without departing from the basic concept and scope of theinvention.

1. An exhaust gas purification device comprising a substrate used forpurifying components contained in an exhaust gas discharged from anengine, the substrate having partitions which define passages and areformed of porous material having fine pores each having a predeterminedaverage size, the end portions of the adjacent partitions defining eachof part of the passages of the substrate being partially connected toeach other such that the end portions are tapered toward the outside ofthe substrate, the tapered end portions partially closing the endopening of alternating corresponding passages and forming a small holein the alternating corresponding passages defined by the tips of thetapered end portions, and the size of each small hole being smaller thanthe cross sectional area of each of the alternating correspondingpassages and larger than the sizes of the fine pores of the partitions.2. An exhaust gas purification device as set forth in claim 1, whereinthe end portions of the adjacent partitions defining each of part of thepassages of the substrate are partially connected to each other at theirupstream ends such that the end portions are tapered toward the outsideof the substrate, and the end portions of the adjacent partitionsdefining each of remaining passages of the substrate are partiallyconnected to each other at their downstream ends such that the endportions are tapered toward the outside of the substrate.
 3. An exhaustgas purification device as set forth in claim 2, wherein the tapered endportions and the remaining partitions carry oxidation material foroxidizing the particulates, and the amount of the oxidation materialcarried by each upstream tapered end portion per unit volume is largerthan that carried by each downstream tapered end portion per unitvolume.
 4. An exhaust gas purification device as set forth in claim 1,wherein the end portions of the adjacent partitions defining each ofpart of the passages of the substrate are partially connected to eachother at their upstream ends such that the end portions are taperedtoward the outside of the substrate, and the end portions of theadjacent partitions defining each of remaining passages of the substrateare connected to each other at their downstream ends such that the endportions are tapered toward the outside of the substrate and thedownstream end opening of the passage is completely closed.
 5. Anexhaust gas purification device as set forth in claim 1, wherein the endportions of the adjacent partitions defining each of part of thepassages of the substrate are partially connected to each other at theirdownstream ends such that the end portions are tapered toward theoutside of the substrate, and the end portions of the adjacentpartitions defining each of remaining passages of the substrate areconnected to each other at their upstream ends such that the endportions are tapered toward the outside of the substrate and theupstream end opening of the passage is completely closed.
 6. An exhaustgas purification device as set forth in claim 1, wherein the substrateis used as a particulate filter arranged in an exhaust gas passage of anengine for collecting particulates contained in an exhaust gasdischarged from an engine.
 7. An exhaust gas purification device as setforth in claim 6, wherein the tapered end portions carry oxidationmaterial for oxidizing the particulates.
 8. An exhaust gas purificationdevice as set forth in claim 7, wherein the amount of the oxidationmaterial carried by each tapered end portion at its upstream surface perunit are is larger than that at its downstream surface per unit area. 9.An exhaust gas purification device as set forth in claim 7, wherein aprocess for increasing the temperature of the filter is performed. 10.An exhaust gas purification device as set forth in claim 7, wherein thefilter carries a NOx carrying agent to take in and carry the NOx thereinwhen excessive oxygen exists therearound, and to discharge the carriedNOx therefrom when the concentration of the oxygen decreases.
 11. Anexhaust gas purification device as set forth in claim 7, wherein thefilter carries a precious metal catalyst.
 12. An exhaust gaspurification device as set forth in claim 11, wherein the oxidationmaterial is an active oxygen production agent to take in and carry theoxygen when excessive oxygen exists therearound, and to discharge thecarried oxygen therefrom in the form of active oxygen when theconcentration of the oxygen decreases, and the active oxygen productionagent discharges the active oxygen therefrom when the particulatesadhere to the filter to oxidize the particulate adhering to the filterby the active oxygen.
 13. An exhaust gas purification device as setforth in claim 12, wherein the active oxygen production agent comprisesone of an alkali metal, an alkali earth metal, a rare earth and atransition metal.
 14. An exhaust gas purification device as set forth inclaim 12, wherein the active oxygen production agent comprises one of analkali metal and an alkali earth metal having an ionization tendencyhigher than that of calcium.
 15. An exhaust gas purification device asset forth in claim 12, wherein the air fuel ratio of at least part ofthe exhaust gas flowing into the filter is temporarily made rich tooxidize the particulates adhering to the filter.
 16. An exhaust gaspurification device as set forth in claim 6, wherein an oxidation meansfor oxidizing components contained in the exhaust gas is arranged in theexhaust gas passage of the engine upstream of the filter.
 17. An exhaustgas purification device as set forth in claim 16, wherein the oxidationmeans is an oxidation catalyst.
 18. An exhaust gas purification deviceas set forth in claim 16, wherein the oxidation means is a NOx catalystto carry the NOx when the lean exhaust gas flows thereinto and to reducethe carried NOx when the rich exhaust gas flows thereinto.
 19. Anexhaust gas purification device as set forth in claim 6, wherein thesize of each small hole of the filter at the low temperature region ofthe filter is larger than that at the high temperature region of thefilter.
 20. An exhaust gas purification device as set forth in claim 19,wherein the low temperature region is the peripheral region of thefilter, and the high temperature region is the central region of thefilter.
 21. An exhaust gas purification device as set forth in claim 19,wherein the cross sectional area of each passage of the filter at thelow temperature region of the filter is larger than that at the hightemperature region of the filter.
 22. An exhaust gas purification deviceas set forth in claim 6, wherein the cross sectional area of eachpassage of the filter at the low temperature region of the filter islarger than that at the high temperature region of the filter.
 23. Anexhaust gas purification device as set forth in claim 22, wherein thelow temperature region is the peripheral region of the filter, and thehigh temperature region is the central region of the filter.
 24. Anexhaust gas purification device as set forth in claim 22, wherein thesize of each small hole of the filter at the low temperature region ofthe filter is larger than that at the high temperature region of thefilter.
 25. An exhaust gas purification device as set forth in claim 6,wherein an exhaust gas purification means for purifying componentscontained in the exhaust gas is arranged in the exhaust gas passage ofthe engine downstream of the filter.
 26. An exhaust gas purificationdevice as set forth in claim 25, wherein the exhaust gas purificationmeans is a NOx catalyst to carry the NOx when the lean exhaust gas flowsthereinto, and to reduce the carried NOx when at least the generallystoichiometric exhaust gas flows thereinto.
 27. An exhaust gaspurification device as set forth in claim 25, wherein the exhaust gaspurification means is an additional particulate filter which can oxidizethe particulates contained in the exhaust gas.
 28. An exhaust gaspurification device as set forth in claim 25, wherein the filter isarranged at least near the exhaust manifold.
 29. An exhaust gaspurification device as set forth in claim 25, wherein the device furthercomprises a bypass passage which extends from the engine exhaust gaspassage between the filter and the exhaust gas purification means to theexhaust gas passage of the engine downstream of the exhaust gaspurification means to bypass the exhaust gas purification means, and aswitch valve for switching the flow of the exhaust gas into the exhaustgas purification means and into the bypass passage, the filter carries aSOx carrying agent to carry the SOx when the lean exhaust gas flowsthereinto, and to release the carried SOx when at least the generallystoichiometric exhaust gas flows thereinto and the temperature of theSOx carrying agent has a temperature higher than a SOx releasetemperature, the switch valve is positioned such that the exhaust gasflows into the exhaust gas purification means when the SOx is notreleased from the SOx carrying agent, and is positioned such that theexhaust gas flows into the bypass passage when the SOx is released fromthe SOx carrying agent.
 30. An exhaust gas purification device as setforth in claim 29, wherein a catalyst for oxidizing the componentscontained in the exhaust gas is arranged in the bypass passage.
 31. Anexhaust gas purification device as set forth in claim 1, wherein thesubstrate is arranged in an exhaust gas passage of an engine, thesubstrate carrying a hydrocarbon collection agent for collectingunburned hydrocarbon contained in an exhaust gas discharged from anengine, and a hydrocarbon purification catalyst for purifying unburnedhydrocarbon, the hydrocarbon collection agent collects unburnedhydrocarbon when the agent has a temperature lower than a hydrocarbonrelease temperature, and releases the collected unburned hydrocarbontherefrom when the agent has a temperature higher than the hydrocarbonrelease temperature, the hydrocarbon purification catalyst purifiesunburned hydrocarbon when the catalyst has a temperature higher than ahydrocarbon purification temperature, the hydrocarbon releasetemperature is set such that the unburned hydrocarbon is released fromthe hydrocarbon collection agent when the hydrocarbon purificationcatalyst has a temperature lower than the hydrocarbon purificationtemperature.
 32. A method for producing a substrate used for purifyingcomponents contained in an exhaust gas discharged from an engine, thesubstrate having a plurality of exhaust gas passages defined bypartitions formed of porous material, the end portions of the partitionsdefining each of part of the exhaust gas passages being partiallyconnected to each other at one end of the exhaust gas passage such thatthe end portions are tapered toward the outside of the substrate anddefine a small hole by the tips thereof, the end portions of thepartitions defining each of the remaining exhaust gas passages beingpartially connected to each other at the other end of the exhaust gaspassage such that the end portions are tapered toward the outside of thesubstrate and define a small hole by the tips thereof, wherein themethod comprises a step of gathering and connecting the end portions ofthe partitions defining each exhaust gas passage to be closed at its endopening, and a step of forming a small hole defined by the tips of theend portions defining each exhaust gas passage to be closed at its endopening, each small hole having a size smaller than the area of the endopening of the corresponding exhaust gas passage and larger than theaverage sizes of the fine pores of the partitions.
 33. A method as setforth in claim 32, wherein the gathering and connecting step and thesmall hole forming step are simultaneously performed.
 34. A method asset forth in claim 33, wherein the gathering and connecting step and thesmall hole forming step are simultaneously performed by pressing adevice having a plurality of projections and pins arranged between theprojections onto the end face of the substrate.
 35. A method as setforth in claim 32, wherein first, the gathering and connecting step isperformed, and then the small hole forming step is performed.
 36. Amethod as set forth in claim 35, wherein in the small hole forming step,the tips of the end portions connected to each other are shaved to formthe small hole.